Polymorphonuclear neutrophilic leukocyte chemotaxis was examined in vitro in the presence of the following antibiotics: tetracycline, chloramphenicol, erythromycin, gentamicin, methicillin, sulfadiazine, cephalothin, oxacillin, nafcillin and clindamycin. Results indicate that high concentrations (1 mg/ml) of tetracycline, nafcillin, erythromycin and sulfadiazine inhibited the chemotactic response of neutrophils. Tetracycline produced an inhibition at concentrations as low as 1mug/ml. A stimulation of the neutrophil chemotactic response was observed in the presence of chloramphenicol in all concentrations. The results are discussed with particular emphasis on the implications to host immunity.