Multiple breakpoint method for measuring effect of antibiotics on endocarditis strains of streptococci. The activity of penicillin alone and combined with aminoglycoside on endocarditis strains of streptococci was examined. Good assay reproducibility was obtained by the use of logarithmic phase cultures standardised by opacity, careful inoculation of well-plates, removal of antibiotic by membrane transfer and incubating survival counts in hydrogen plus carbon dioxide. The use of 10-fold intervals for penicillin concentration simplified assay design without loss of efficiency. 