The study of interspecific gamete interactions has been improved by the use of in vitro fertilization techniques. Zona pellucida is at the cellular level the main barrier to cross-fertilization. Moreover, there are several cases in which the heterologous spermatozoa fail to enter the cytoplasm of zona-free oocytes, emphasizing the notion that another important barrier exists to cross-fertilization at the egg plasma membrane level. We report here a simple method to fertilize hamster oocytes with zona pellucida by human spermatozoa. We used lysophosphatidylcholine (LPC), a fusogenic agent, to treat spermatozoa, spermatozoa and zona-intact oocytes, or zona-intact oocytes alone. This molecule allows the penetration of hamster oocytes zona by human spermatozoa. Our results suggest that LPC modifies the zona pellucida of oocytes and the plasma membrane of both gametes in vitro, favouring sperm penetration through the zona pellucida and gamete fusion.