Neutrophil functions were evaluated in six normal subjects who had received 300 mg of clindamycin orally four times each day for two days. The mean serum concentration was 1.0 mg/l at the time of neutrophil collection. Clindamycin increased phagocytosis of a clindamycin-resistant staphylococcus from 0.8 to 1.0 organism per polymorphonuclear leukocyte (P less than 0.05). The proportion of killed intracellular bacteria increased from 23.8% to 29.8%. Zymosan-induced chemiluminescence was reduced from 130 X 10(3) to 86 X 10(3) counts per 0.2 min (P less than 0.05). Chemotaxis was unaffected. These preliminary results demonstrate measurable effects of clindamycin on neutrophil functions in the host; however, further studies are needed in order to confirm the observed changes.