It is generally believed that Lancefield group A hemolytic streptococci are involved in all or part of the pathogenesis of rheumatic fever.1Although penicillin is an unsatisfactory therapeutic agent for this disease,2this antibiotic should be excellent for prophylaxis of recurrences because of its ability to control the hemolytic streptococcus in vivo. This use of penicillin has been "mentioned hopefully" in the literature since 1946.3The relative infrequency of complications and sequelae are obvious advantages of penicillin in addition to its low toxicity in large dosages.3aSince penicillin, like the sulfonamide drugs, apparently fails to influence the onset and course of an attack of rheumatic fever if given after the preceding hemolytic streptococcic infection,3ait becomes mandatory to eliminate hemolytic streptococci entirely during the period preceding a possible recurrence. Massell, Dow and Jones4reported a lowered incidence of recurrences of rheumatic fever when they