as an example of a typical embodiment we describe an immunoradiometric assay ( irma ). in the assay to be described a sheep anti - interferon antibody is attached to a solid phase , polystyrene , and serves to anchor the interferon present in the sample to the solid phase . the bound interferon is then detected by the addition of 125 i - nk2 ( monoclonal anti - hu - ifnα ) and measurement of the counts bound to the solid phase . the antibody nk2 was prepared as described by d . s . secher and d . c . burke in nature , 285 at pages 446 to 450 ( 1980 ), and in international patent application no . pct / gb 81 / 00067 . three forms of polystyrene have been used as the solid phase . ( 1 ) 3 ml test tubes ( lp3 , luckham , ltd ., bungers hill , u . k .) in the first case the whole tube was counted to measure the bound 125 i - nk2 . when trays were used the bottom of each well was cut off with a hot wire and transformed to a clean tube for counting . in the third case the beads were incubated in 20 - or 60 - well trays ( 93 - 0402 , abbott laboratories , basingstoke , u . k .) and transferred to tubes for counting . the incubation volumes used were 1 ml or 0 . 1 ml ( wells ) and 0 . 2 ml ( beads ). all three supports were found to be satisfactory and for assaying large numbers of samples the beads were preferred for their convenience . nk2 antibody was purified from the serum and ascites fluid of mice carrying nk2 tumours by ammonium sulphate precipitation and deae ion - exchange chromatograph , labelled by the chloramine - t method and desalted on sephadex g - 50 ( fine ) column . the labelled igg had a specific activity of about 2 ci / μ mole or about 1 atom 125 i per molecule igg . in each assay a standard curve was constructed using either the interferon reference standard mrc 69 / 19 or a laboratory standard . in one such assay ( see fig1 ) igg was purified from a sheep anti - hu - ifnαantiserum ( 450 , 000 neutralising units / ml ) by ammonium sulphate precipitation and deae - cellulose ion exchange chromatography and coated on to polystyrene beads by incubation of the beads at 4 ° c . for 16 hrs . in the sheep igg ( 20 μg / ml in pbs -- phosphate buffer solution --, 5 mm edta , 0 . 1 % nan 3 ). several hundred beads were coated , washed in a medium ( hs medium ) consisting of pbs , 10 % horse serum , 0 . 1 % nan 3 and stored in hs medium at 4 °. assay trays ( 20 - or 60 - well , abbott ) were incubated with hs medium at 4 ° to block any sites for protein attachment . interferon samples were diluted in hs medium and duplicate samples ( 200 μl ) added to antibody - coated beads . after 4 h at 4 ° the beads were washed with 12 ml hs medium each using a combined dispenser / aspirator (&# 34 ; pentwash &# 34 ;, abbott laboratories ). after removal of any residual medium , 125 i - nk2 ( purified igg , 40 , 000 cpm ) was added and incubated for 16 h at 4 °. the beads were washed as before and transferred to a gamma - counter . the broken line indicates the background cpm bound when the coated beads were incubated in hs medium without interferon . the counts bound at 8 , 000 u / ml were at a maximum ; no further increase was observed with interferon concentrations up to 10 6 u / ml . the low level of non - specific binding (& lt ; 1 % of input counts is an important characteristic of the sandwich assay ). in a second experiment ( see fig2 ) we showed that at low interferon concentrations the counts bound are proportional to the interferon concentration . interferon samples were prepared by dilution of the reference research standard in hs medium and assayed as described for fig1 except that only 23 , 000 cpm 125 i - nk2 was added to each bead . the dotted line indicates the background cpm bound and was the mean of eight values ( s . e .=± 6 cpm ). it can be seen that concentrations of ≧ 50 u / ml can be easily measured . the standard assay conditions ( i . e . those used in the construction of fig1 ) have been adjusted for the convenient measurement of samples encountered in the purification of interferon . such samples have values of 10 3 - 10 7 u / ml . solutions containing 10 3 u / ml are diluted by serial dilution and the titration curve obtained matched to the standard curve . comparison of the above immunoassay technique with the known antiviral assay technique on the same samples shows good agreement , with experimental error . the immunoassay of the present invention offers considerable advantages over the conventional biological assay . since the antibody - coated polystyrene can be used at 4 °, samples can be assayed at short notice and the results are obtained within 24 hours of beginning the assay . the standard curves show very little variation from assay to assay compared to the much greater inherent variability of biological assays . four measurements of a solution of 2000 u / ml gave a value of 3063 ± 345 cpm in independent assays in which the input cpm ranged from 47000 to 53000 cpm . only small amounts of sheep antibody are necessary , since the igg solution used to coat the polystyrene can be re - used without apparently reducing the sensitivity of the assay . the quality of the sheep antibody is not a critical factor in the success of the assay , and other antibodies to hu - ifnα , as described above , may substitute equally well . the assay exploits the specificity of the monoclonal antibody nk2 and since this is the product of a hybrid myeloma cell line in culture it can be produced in large amounts without any change in quality . finally the assay is inexpensive ( especially since all the plastic ware except the beads may be re - used ), lends itself to automation and it is possible for one person to assay several hundred samples in a day . a limitation in the conventional assay of interferon in serum or other biological fluids has been the need to dilute the sample until the effects of other non - interferon substances that affect viral growth no longer mask the action of the interferon . dilution of a sample of interferon purified by affinity chromatography on nk2 - sepharose in either pbs , 10 % horse serum , 0 . 1 % nan 3 or in undiluted normal human serum resulted in the identical titration curves shown in fig3 indicating that the immunoassay of the invention overcomes this problem . in this experiment interferon was ( a ) diluted in hs medium or ( b ) undiluted normal human serum and samples assayed as described in relation to fig1 except that 3 ml test tubes were used as the polystyrene support rather than the beads . the solid line represents the cpm bound in case ( a ) and the broken line the results in case ( b ). the lower limit in the measurement of serum interferon by biological assays has been sufficient to detect interferon in the serum of some patients to whom interferon has been administered . this sensitivity is insufficient , however , to measure the interferon concentration ( if any ) in normal human serum . when conditions are adjusted to measure concentrations of interferon as described above the immunoassay of the invention can reliably measure around 50 u / ml . the sensitivity of the assay can moreover be further increased by prior concentration and purification of the interferon on a small nk - 2 - affinity column .