Fifty adult patients were treated with 0.6 to 1.8 grams/day of parenteral clindamycin-2-phosphate for 4 to 37 days. Favorable clinical results were observed in patients with pneumococcal pneumonia, streptococcal pharyngitis, acute staphylococcal and/or streptococcal soft tissue infection, staphylococcal osteomyelitis, and actinomycosis. Three patients with mycoplasma pneumonia did not respond. Serum concentrations of clindamycin, particularly after intravenous administration, were many-fold higher than the minimum inhibitory concentrations of the clinical isolates. Patients' clindamycin-containing sera showed antibacterial activity against their own infecting pathogens in high dilution. Eight to 28 per cent of the administered clindamycin was excreted in the urine in active form. Clindamycin was well tolerated and generally nontoxic although reversible rises in SGOT and SGPT were frequently observed without other laboratory or clinical evidence of hepatotoxicity. In animal experiments simulating clinical use, wide tissue distribution of clindamycin was observed and there was no histologic evidence of hepatic cell injury.