The aim of this study was to determine the effects of preincubation in peritoneal fluid on the follicular fluid-induced acrosomal reactivity of human spermatozoa in vitro. Thirty women participating in our IVF-ET program were given a GnRH-analogue, highly purified FSH and hCG in order to induce superovulation. Peritoneal and follicular fluids were aspirated during pick-up laparoscopy, centrifuged, filtered and frozen until use. An aliquot of swim-up suspension from normospermic semen specimens (n = 30) was incubated with peritoneal fluid or HAM-F10 for 30-180 min, and follicular fluid (in volumetric proportion approximately 50/50 with peritoneal fluid) was subsequently added. The percentage of acrosomally-reacted spermatozoa was assessed using the FITC-conjugated Pisum sativum lectin before and after incubation in peritoneal fluid or control medium, as well as after follicular fluid addition. Peritoneal fluid was not able to stimulate acrosomal reactivity; further, preincubation in peritoneal fluid decreased, but not abolished, the follicular fluid-induced acrosomal reactivity. A longer pre-incubation in peritoneal fluid was associated with a lower percentage of reacted spermatozoa in response to the addition of follicular fluid. In conclusion, our data suggest that peritoneal fluid acts maintaining spermatozoa in an unreacted status in the upper female genital tract. After mixing with follicular fluid, a phenomenon that is likely to occur at ovulation, peritoneal fluid reduces, but does not abolish, the stimulating effect of follicular fluid on acrosomal reactivity.