Leucocytospermia has been associated with reduced sperm motility and decreased capacity for sperm-egg interaction. This effect could be mediated through reactive oxygen species (ROS), which, at high concentrations, induce lipid peroxidation and cellular death. The high impact on sperm capacitation reported in other mammalians should be more accurately assessed in the human because premature activation could affect sperm fertilizing capacity. The aim of this study was to evaluate both the effect of ROS on sperm capacitation and the protective role of seminal plasma. Spermatozoa selected by Percoll gradient were incubated with polymorphonuclear (PMN) granulocytes isolated from blood and activated by phorbol-12-myristate 13-acetate (PMA). Different seminal plasma concentrations were added immediately or after 3-h incubation. Afterwards, ROS production was evaluated by luminescence and sperm capacitation by chlortetracycline stain. In PMN granulocytes and sperm suspensions, the basal ROS production was < 32 x 103 relative luminescence units (RLU). After stimulation with PMA, the rate of ROS production by PMN increased to 1,287 x 103 RLU. Incubation of sperm with activated PMN resulted in an increase of sperm capacitation (37% versus 19% in the control). Immediate addition of seminal plasma caused a significant reduction in ROS (P < 0.01) and prevented sperm from capacitating. A higher effect in inhibition of sperm capacitation was observed when seminal plasma had been added after 3-h incubation. The results suggest that human sperm capacitation can prematurely be induced by exogenous ROS and this effect can be reversed by seminal plasma. Thus, human sperm capacitation is another functional parameter that may be affected by nonphysiological ROS production.