Antiproteinase sera were prepared by immunizing horses with filtrates from a selected strain of group A streptococcus. This strain, which produced high titred proteinase but no erythrogenic toxin, was selected from forty-two strains of group A streptococci which produced varying amounts of proteinase. A few strains belonging to groups B, C, and G were also tested; they were all proteinase-negative. Methods are described for titrating streptococcal proteinase in crude culture filtrates and for measuring the antiproteinase activity of serum.

The antiproteinase titres of sera from immunized horses ranged from 125 units to 1,000 units per cc. in contrast to the low titres of normal horse sera, only 5 per cent of which had titres as high as 10 to 30 units per cc.

The available evidence suggests that the antiproteinase activity of immune sera is dependent on the action of specific antibody for streptococcal proteinase. Patients infected with group A streptococci do not develop high anti-proteinase titres. There appears to be no correlation between the occurrence of rheumatic fever and the antiproteinase titre of the patient's serum.

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