The effect of the antiseptic povidone-iodine (P-I) on neutrophil locomotion was tested in vitro. Neutrophil migration into Micropore filters was significantly stimulated by P-I at concentrations between 0.03% and 0.005%. At higher concentrations (greater than or equal to 0.05%), a dose-dependent inhibition of neutrophil migration could be observed which was due to cytotoxic effects of P-I as shown by pyknosis and cell lysis. To analyze these effects, 2 components of P-I (namely, povidone and iodide) were tested separately. In these tests, povidone induced a pronounced stimulation of neutrophil migration at concentrations similar to the stimulatory concentrations of P-I. Inhibition of neutrophil migration or cytotoxic effects of povidone was not observed, even when tested at high concentrations (5.0%). Iodide (as NaI or KI) was cytotoxic and strongly inhibited neutrophil migration when it was tested at concentrations greater than that likely to be present in the inhibitory concentrations of P-I. Stimulatory effects of iodide on neutrophil migration could not be observed when tested over a wide range of noncytotoxic concentrations. Free iodine was not tested, but was considered to be the toxic component by exclusion. The patterns of response were similar for neutrophils from the 4 species tested. Migration of rabbit neutrophils, however, was inconsistently and weakly stimulated by P-I or povidone. These data indicate that the widely used antiseptic P-I, depending on its concentration, can either stimulate or inhibit neutrophil migration.