Male infertility can be related to defects in motility, capacitation, acrosome reaction, binding and penetration of the zona pellucida. While different in-vitro techniques (such as micromanipulation which is complicated and expensive) are available for the treatment of male infertility, several pharmacological agents have been shown to increase fertilizing capacity under accurate experimental conditions. Gastrin-releasing peptide (GRP, the mammalian homologue of the amphibian skin peptide bombesin) is present in the reproductive tract and expressed by the pregnant ovine endometrium prior to attachment and throughout the pregnancy. A bombesin-like peptide resulting from alternate splicing of the GRP gene in testis has been detected in primates. In this study, we have tested the ability of GRP to enhance human sperm functions such as motility, capacitation, zona binding and acrosome reaction. Analysis of sperm motility was performed with the ATS 20 computer-assisted semen analysis (CASA) system. Zona binding was analysed using intact human unfertilized oocytes and selective labelling of spermatozoa with two fluorochromes. Our results did not show any positive effect of GRP on these parameters under our experimental conditions. However, when GRP at the concentration of 100 nM was added after ionophore treatment, the percentage of reacted cells increased. significantly (P < 0.05) compared with situations where each agent was used alone. This led us to suppose that the role of bombesin in the different stages of fertilization might not exclude other unknown factors.