Human oocytes that failed to fertilize in a clinical in vitro fertilization (IVF) program were freed from the zona pellucida and reinseminated with spermatozoa from subfertile patients or from fertile donors. Penetration results were, in general, significantly worse when patients spermatozoa were used as compared with donor spermatozoa. When inseminated with donor spermatozoa, oocytes from cases of male infertility gave better penetration results as compared with those from cases of idiopathic infertility with no apparent sperm defect. No differences in penetration results were found between oocytes aged 1 day and 2 days in culture before zona removal and reinsemination, even though less pronuclei reached full functional maturity in the more aged group of eggs. These results indicate that human zona-free eggs prepared from oocytes after unsuccessful IVF attempts may be used to assess the fertilizing ability of human spermatozoa. Moreover, when used with donor spermatozoa, this system may help to determine the contribution of the oocyte to fertilization failure.