Polymorphonuclear leukocyte (PMNL) chemotaxis was evaluated in ten healthy volunteers who had received 600 mg of clindamycin intramuscularly. Serum obtained 3 hours after the administration of clindamycin significantly increased PMNL chemotaxis. Serum obtained at 12 and 24 hours after the administration of the drug did not induce significant increase in PMNL chemotaxis. The administration of clindamycin had no direct effect on the PMNLs in terms of their chemotactic activity. These results demonstrate serum-associated augmentation of PMNL chemotaxis by clindamycin in vivo which may be of potential clinical benefit in the outcome of infections.