The zona-free hamster egg penetration test was performed on 74 random consecutive semen samples from men of infertile couples and on 7 men of proven fertility (semen donors). Of these 74 men, 31 had never before delivered a semen sample (Group I), while the remaining 43 had delivered samples before (Group II). The penetration rates were correlated with the parameters of the semen analysis. In Group I, the mean penetration rate was 59%, in Group II, 42%, and among the fertile donors, 52%. No strong correlation was found between hamster egg penetration rates and the various semen parameters. Only the proportion of motile spermatozoa in Group II correlated significantly with the egg penetration test (P less than .05). Differences in methodology make comparison with the results from different laboratories difficult. It is likely that the egg penetration test measures some separate quality of the spermatozoa that may not be clear from the routine semen analysis. The role of the hamster egg penetration test in the investigation of the causes of infertility should be evaluated further.