FLEMING,1 in his original studies on the antibacterial action of cultures of his strain of Penicillium, included some strains of Streptococcus viridans, which he found to vary considerably in their susceptibility. Others2345678 have since confirmed these findings with more purified preparations of penicillin in vitro, noting the wide differences in susceptibility of single strains and finding some to be relatively resistant. Clinical trials in the treatment of subacute bacterial endocarditis were begun in 1942. The earliest reports were somewhat discouraging,910111213141516 but they did suggest the possibility that larger doses of penicillin given over a longer time might produce better results. . . .