Since progesterone has been claimed to induce acrosomal reaction and hyperactivated motility of human spermatozoa, the present study was undertaken to determine if its presence at concentrations similar to those of peri-ovulatory follicular fluid could influence the effect of peritoneal fluid on sperm motility in vitro. To this end, 11 sperm samples were incubated at 37 degrees C with five peritoneal fluids with/without exogenous progesterone, and sperm motility was assessed using a computer-assisted analyser at time (t) = 0, 2.5, 5 and 24 h. Overall there was no observable constant trend for enhancement or inhibition of sperm motility. Progesterone generally induced a negative effect on those sperm samples with high velocities in the native peritoneal fluids and a positive effect on those sperm samples demonstrating low motility in the native peritoneal fluids. The incorporation of progesterone into the incubation medium seemed to result in a 'tuning' of sperm velocity to around 30-50 micron/s. However, a given sperm sample reacted differently when incubated with various peritoneal fluids and, reciprocally, different semen samples incubated with the same peritoneal fluid showed very variable motility patterns. The greater variability of the effects exerted by progesterone on sperm motility could arise from the fact that each sperm sample may contain subpopulations of gametes with different sensitivity to progesterone.