an improved method of preparing acid addition salts of dl 6 - phenyl - 2 , 3 , 5 , 6 - tetrahydroimidazothiazole by reacting dl - 3 -- 2 - iminothiazolidine or an acid - addition salt thereof , with a mixture of concentrated hydrochloric acid and concentrated sulfuric acid to obtain dl 3 -- 2 - iminothiazolidine acid - addition salt , treating this compound with aqueous alkali to obtain dl 6 - phenyl - 2 , 3 , 5 , 6 - tetrahydroimidazothiazole free base , and converting the latter compound to dl 6 - phenyl - 2 , 3 , 5 , 6 - tetrahydroimidazo thiazole acid - addition salts in a highly purified state . the products are useful as anthelmintics .