OBJECTIVE Successful fertilization in humans follows a complex series of events, including the completion of meiotic maturation of the oocyte with the extrusion of the second polar body, the decondensation of the sperm nucleus and the maternal chromosomes into male and female pronuclei, the restoration of the sperm centrosome, and the nucleation of microtubule-mediated motility necessary to bring the male and female pronuclei into close apposition. These events occur after both fertilization in vitro and after intracytoplasmic sperm injection (ICSI), a new technique which is currently being applied in many clinics to overcome severe male infertility. Defects in any of the events leading to fertilization can be lethal to the zygote and may prove to be causes of infertility. METHODS Imaging of inseminated human and rhesus oocytes using immunohistochemical techniques reveals several phases at which fertilization arrests. RESULTS Oocytes from some infertile patients failed to complete fertilization due to failure of the sperm aster microtubules in uniting the sperm and egg nuclei. The rate of sperm aster formation, size, and organization during fertilization has been used as a measurement of bovine sperm quality. The development of an assay using Xenopus laevis oocyte extract can also be used to test sperm from various species for their ability to form esters and perform other centrosomal functions in vitro, as well as another indicator of sperm quality. Semen from men with questionable fertility was found to contain sperm which are generally incapable of producing sperm asters. In addition, the activity of centrosomal proteins such as gamma-tubulin and centrin have been detected in mammalian eggs and sperm. The levels of gamma-tubulin increase markedly after exposure to X. laevis egg extract. CONCLUSIONS Defects in either male or female nucleus decondensation also resulted in the arrest of fertilization and was found to occur in both inseminated human oocytes and in rhesus oocytes fertilized by ICSI. These discoveries on the molecular basis of infertility in humans have important implications for infertility diagnosis and managing reproduction.