In an analysis of 38 proven-fertile donor samples and 84 patient samples of either unproven fertility or suspected infertility, the number of hamster ova penetrated by human sperm (% HOP) was significantly different (p less than 0.05) between the donor and suspected-infertile groups. Lowest percent HOP rates; which were recorded for patients with oligozoospermia (less than 20 X 10(6) sperm per milliliter), were significantly different from donor rates (p less than 0.01). Sperm samples from 15 patients of unknown fertility were incubated with human eggs and were simultaneously incubated in the hamster ova-sperm penetration assay. All 13 patients fertilizing at least one human ovum also penetrated hamster ova at greater than 10%. The two samples failing to fertilize any human ova had one HOP above 10% and one below 10%. Interassay ranges were low over short periods of time (weeks), but the range was high (41%) for one individual when the SPA was performed months later. Fluctuations in % HOP over extended periods of time may be due to changes in male fertility status and not to unreliability of the assay.