The probable relationship of Lancefield group A hemolytic streptococcic disease to rheumatic fever has led to the therapeutic trial of various antistreptococcic substances in this disease. Swift1 and Jones2 have described the effects of sulfanilamide in acute rheumatic fever. Swift3 and his co-workers4 have also reported the results of streptococcic antiserums in.this disease. Both of these entirely unrelated substances were found to be of no therapeutic value, and the evidence is suggestive that at times they may have been harmful. In view of these observations, when penicillin became available there was little cause for optimism that it would be beneficial in this disease. However, because of the great need for a more satisfactory therapeutic agent for rheumatic fever and the knowledge that the value of penicillin in this disease would eventually need to be determined, it was felt that an organized study by producing an answer