The chemotactic response of human neutrophils was examined in the presence of tetracycline. High tetracycline concentrations decreased chemotaxis in vitro but had no inhibitory effect on ability of neutrophils to respond to the chemotactic components of human complement. Human serum and endotoxin mixtures decreased chemotactic ability when they were preincubated with various tetracycline concentrations, the decrease was concentration dependent. This inhibitory effect on the human neutrophil chemotactic response was not time dependent. Eight different tetracycline compounds apparently produced similar inhibitory effects on the human neutrophil chemotactic response in vitro. Tetracycline may inhibit the generation of neutrophil chemotactic factors of human serum or bind with chemotactic factors after their generation to cause inactivation.