Intravenous inoculation of a group A hemolytic streptococcus caused lethal infections in all of eight untreated monkeys. Intramuscular injections of clindamycin-2-phosphate in a daily dose of 25 mg/kg given in equal morning and afternoon doses for 10 days resulted in survival of all of eight monkeys. Similar results were observed with the same dose schedule of clindamycin hydrochloride given intragastrically; no fatalities occurred among eight monkeys. In monkeys receiving erythromycin stearate intragastrically or methicillin intramuscularly, three of eight and four of eight monkeys, respectively, died. Duration of both illness and positive blood cultures was greater in the erythromycin- and methicillin-treated survivors than in the clindamycin-treated monkeys. The differences in results between clindamycin and erythromycin could not be correlated with serum antibacterial activity levels, which were similar, or with minimal inhibitory concentrations, which were 0.02 mug/ml with both antibiotics. With methicillin, however, the minimal inhibitory concentration was 0.16 mug/ml and serum antibacterial activity varied from titers of less than 1:2 to 1:8. As in previous studies of staphylococcal infections in monkeys with the same antibiotics, in vitro susceptibility data and serum antibacterial activity did not completely correlate with in vivo results.