in order to obtain a uniform hgh bulk material a solid hgh preparation was prepared essentially free of excipients . to obtain this , freeze - drying was carried out at low ph ( approximately 5 ) close to the isoelectric point of hgh . before freeze - drying the ph was adjusted with acetic acid , thus the only counter ion present was acetate buffering at ph 5 . freeze - drying at fairly low ph has the advantage that the formation of degradation products are minimised and a more homogeneous product is obtained in comparison with freeze - drying carried out at higher phs such as in the carbonate containing systems . the starting material for the production of solid bulk hgh is the product from the last step of the purification process used for purifying recombinantly produced hgh . the process steps are designed to transform the hgh solution into a freeze - dried powder without or almost without excipients . the hgh solid bulk preparation consists of a concentration step , a desalting step , a microfiltration step , an isoprecipitation step , and a freeze - drying step . the concentration step : the step has been introduced in order to concentrate the hgh solution from , for example , 2 - 6 mg / ml to 20 - 70 mg / ml , and to obtain a buffer change from , for example , 7 m urea buffer to 0 . 35 m ammonium acetate buffer . the concentration is performed on an anion exchange column . the desalting step : the step has been introduced in order to obtain hgh in pure water without salts and other excipients . the desalting is performed on a desalting column . the microfiltration step ( 0 . 22 μm filter ): the step has been introduced in order to minimise the microbial contamination . sterility is not claimed . the isoprecipitation step : the step has been introduced to stabilise the product during the final freeze - drying . the solution was precipitated by adjusting the ph to 4 . 7 - 5 . 0 with 1 m of acetic acid ( and eventually ammonia , if necessary to further adjust the ph ). the freeze - drying step : the step has been introduced in order to obtain a hgh solid bulk powder that is nearly 100 % somatropin and which is easy to dissolve in aqueous buffer . all finished hgh products can be manufactured from the hgh solid bulk of the present invention . hgh prepared as described above will result in a bulk product c ) having a total content of polymeric and dimeric forms of hgh of about 0 . 5 to about 2 %; d ) having a content of sulphoxide forms of hgh of from about 0 . 3 to about 0 . 8 %; f ) having a content of desamido forms of from about 0 . 6 % to about 1 %; and g ) having a low content of other degradation products , such as spitforms , and cyclic imides . the isoelectrical point of hgh is found at a value of ph 4 . 9 , thus a ph value of from 4 . 7 to 5 . 0 may suffice to isoprecipitate the human growth hormone . the following examples are offered as an illustration of the invention , not as a limitation . the starting material was hgh dissolved in 7 m urea buffer at a hgh concentration of 2 - 6 mg / ml . 1 ) the solution was desalted on a desalting column followed by 3 ) the concentrated solution was microfiltrated through a 0 . 22μ filter to obtain a clear solution with a reduced microbial level . 4 ) the filtered solution was desalted on a g25 sephadex column in water resulting in an excipient - free hgh solution with ph about 7 . 5 ) the hgh solution was freeze - dried to obtain hgh powder with a residual water content of from about 0 . 5 to about 2 %. the hgh powder obtained had a high content of dimeric hgh (= 2 . 2 %) the freeze - dried powder was fluffy and electric and very difficult to handle as it generated hgh dust to the environment . the starting material was the same as described in example 1 . 1 ) the solution was desalted on a desalting column followed by 3 ) the concentrated solution was microfiltrated through a 0 . 22μ filter to obtain a clear solution with a reduced microbial level . 4 ) the filtered solution was applicated on a g25 sephadex column in water to obtain an excipient - free hgh solution . 5 ) the product was isoprecipitated at ph 4 . 9 by adjusting with 1 m hcl solution . the isoprecipitated hgh was isolated by centrifugation . 6 ) the centrifuged hgh was freeze - dried to a residual water content of from about 0 . 5 to about 2 %. the hgh powder obtained had a satisfactory content of dimeric hgh (= 0 . 8 %). the sulphoxide content of 0 . 9 % was considered to be too high . furthermore , the loss of product (= 40 %) during the centrifugation was unacceptable high . the starting material was the same as described in example 1 . in order to increase the process yield during the isoprecipitation step and centrifugation step ethanol was added to decrease the solubility . 1 ) the solution was desalted on a desalting column followed by 3 ) the concentrated solution was microfiltrated through a 0 . 22μ filter to obtain a clear solution with a reduced microbial level . 4 ) the filtered solution was applicated on a g25 sephadex column in water to obtain an excipient - free hgh solution . 5 ) ethanol was added to the desalted aqueous solution of hgh to a concentration of from about 28 to about 30 % of the final solution . 6 ) the product was isoprecipitated by adjusting the ph to 4 . 9 with 1 m hcl solution . the isoprecipitated hgh was isolated by centrifugation . 7 ) the centrifuged hgh was freeze - dried to a residual water content of from about 0 . 5 to about 2 %. the hgh powder obtained had an unacceptable high content of dimeric hgh (= 1 . 9 %) and of polymeric hgh (= 4 . 9 %). loss of product during the isoprecipitation and centrifugation was 10 %. the starting material was the same as described in example 1 . in the isoprecipitation step the ph is adjusted with 1 m acetic acid solution . in order to avoid product loss , the combined mixture of isoprecipitated hgh and the supernatant hgh was freeze - dried as a slurry to a residual water content of from about 0 . 5 to about 2 %. 1 ) the solution was desalted on a desalting column followed by 3 ) the concentrated solution was microfiltrated through a 0 . 22μ filter to obtain a clear solution with a reduced microbial level . 4 ) the filtered solution was applicated on a g25 sephadex column in water to obtain an excipient - free hgh solution . 5 ) ethanol was added to the desalted aqueous solution of hgh to a concentration of from about 28 to about 30 % of the final solution . 6 ) the product was isoprecipitated by adjusting the ph to 4 . 9 with 1 m acetic acid solution . 7 ) the combined mixture of isoprecipitated hgh and the supernatant hgh was freeze - dried as a slurry to a residual water content of from about 0 . 5 to about 2 %. the content of dimeric hgh was 0 . 4 %. the content of polymeric hgh was & lt ; 0 . 2 % and the content of sulphoxide forms of hgh was 0 . 4 % which is considered acceptable . the freeze - drying . process was composed of a freezing step , a primary drying step , and a secondary drying step . the freeze - drying process was designed to obtain a desired low level of residual water ( from about 0 . 5 to about 2 %). the solutions to be freeze - dried in the examples does not contain gas - evolving buffer excipients ( for example nh 4 +, co 3 2 − ). therefore , pressure increase during the freeze - drying process was not observed . the hgh powder obtained by freeze - drying of the isoprecipitated slurry resulted in a free - flowing and non - electric powder with good handling properties . surprisingly , the homogeneity and reproducibility of the freeze - drying of a slurry were highly satisfactory .