THE iodimetric estimation of procaine penicillin presents difficulties because of the interaction of procaine with iodine. Sodium penicillin to which an equivalent weight of procaine hydrochloride has been added gives results which may be from 3 to 5 per cent lower than for the untreated penicillin. Although the ferricyanide method of Hiscox1 is said to be free from the above objection, we could not obtain consistent results with it, a spread of ± 4 per cent being obtained in a series of determinations on a sample of sodium penicillin. The removal of the procaine ion by precipitation followed by iodimetric determination of the penicillin seemed the most promising line of attack, and sodium silicomolybdate and sodium silicotungstate were both found suitable for this purpose, although the latter possesses certain advantages over the former and is obtainable commercially. Both substances give with procaine salts heavy precipitates which are granular in the presence of sodium chloride (3 per cent) and can readily be centrifuged to a compact mass permitting decantation of the supernatant liquid. Precipitation is virtually complete, and neither precipitant has any action upon iodine or penicillin.