The sixfold increased incidence of staphylococcic endocarditis since 1953 was due to cases caused by penicillin-G-resistant strains. On the basis of this change in the staphylococcus and other clinical and laboratory findings, the patients were classified into three groups: acute bacterial endocarditis, due to coagulase-positive staphylococci which were usually resistant to penicillin; subacute bacterial endocarditis, due to coagulase-negative organisms sensitive to penicillin; and, postcardiotomy endocarditis, usually due to coagulase-negative strains that were resistant to penicillin. Treatment with the new semisynthetic penicillins gave much more favorable results than with the older antibiotics. Administration of large amounts of the antibiotic for at least six weeks was essential.