Eight years ago, when subacute bacterial endocarditis was first treated with penicillin, clinicians quickly learned that with the small amounts of penicillin employed relapse could be averted only by continuing therapy for many weeks. As penicillin has become more plentiful the average daily dose has been increased and the term of therapy generally fixed at four to eight weeks. Bloomfield1 in a recent report suggests a 30 day treatment schedule, and Keefer and his associates2 advise 8 weeks. In England, Christie3 recommends 500,000 units a day for 28 days, with the expectation that infection will be eradicated in 90% of the patients. The chance observation, in 1946, that a patient who had failed to respond to conventional doses of penicillin administered for many weeks was cured in 11 days with a daily dose of 15 million units, led us to a systematic study of the possibility of