To determine whether secreted neutrophil products affect the migration of motile microorganisms such as Trichomonas vaginalis, stimulated human neutrophils and cell-free oxygen metabolites were used as stimuli in a multiwell filter chemotaxis assay using tritiated T. vaginalis. When stimulated neutrophils were present on the opposite side of the filter, migration of T. vaginalis into the filter was significantly diminished, and this reduction varied with the dose of neutrophil stimulus. The reduction of movement was abrogated by the addition of catalase and superoxide dismutase to scavenge oxygen metabolites. Studies with cell-free hydrogen peroxide or hypochlorite preparations indicated that the reduction in trichomonal trapping in the filter was due to chemorepulsion and not to a nonspecific decrease in motility or adherence. These findings suggest that active migration away from neutrophil products might be a means by which trichomonads avoid the microbicidal functions of host phagocytes.