The effect of platelet-activating factor (PAF) on motility parameters and induction of the acrosome reaction in human spermatozoa was investigated in 36 unselected men with different degrees of initial sperm motility. The characteristics of sperm movement were assessed by computer-assisted sperm analysis (Hamilton-Thorn Motility Analyser) and the percentage of acrosome-reacted spermatozoa was evaluated after 1 h incubation with PAF (10 nM) and staining with fluorescent peanut lectin. We found that short-term (4 h max) incubation with PAF significantly enhanced total and progressive sperm motility as well as acrosome reaction. An increase of sperm motility in response to PAF was present in 16 out of the 25 subjects studied (defined as responders) and was inversely correlated with basal motility. In the 11 samples (six responders and five non-responders) where the incubation with PAF was prolonged overnight, an increase of sperm motility was present in all the subjects studied. Similarly, an increase in numbers of acrosome reactions in response to 10 nM PAF was present in 20 out of the 26 subjects examined, and was inhibited by the PAF receptor antagonist L659 989. Our results indicate a possible physiological role for PAF in fertilization and suggest a potential use of PAF in in-vitro fertilization techniques in cases of reduced sperm motility.