SummaryA series of experiments was carried out in vitro to investigate the activity of erythromycin, used alone or in combination with penicillin, against staphylococci, pneumococci and Group A streptococci at various stages of their growth. The results showed that at concentrations of 1.0μg/ml, 3.0μg/ml and even at the minimum inhibitory concentration of 0.6μg/ml against Staphylococcus aureus SG511, erythromycin produced a marked reduction in bacterial counts. It is suggested that erythromycin and other antibiotics that inhibit protein synthesis may induce anabolic andcatabolic insufficiency inmicro-organisms. When 3.0 fig erythromycin/ml'was combined with 1 unit penicillin/ml, the effect of erythromycin on bacteria previously exposed to penicillin was found to be more satisfactory than that of the opposite sequence. The effect of 0.1μg erythromycin/ml on pneumococci (MIC:0.031μg/ml) was to induce a similar reduction in bacterial counts, as did the combination with 1 unit penicillinjml {MIC: 0.031μg/ml). The effect of 0.2μg erythromycin/ml on the streptococcal counts was weaker, as also was that of 0.2 units penicillin/ml (MICs: 0.031μg/ml, and 0.031 units/ml). When the antibiotics were combined, the results were similar to those obtained with the other bacterial species.