Sperm from some infertile men may consist of several populations.

In the hamster ovum penetration (HOP) test, when ova have equal penetrability and sperm have equal penetrating ability, the distribution of zona-free hamster ova classified by number of penetrating human sperm is expected to follow the Poisson distribution (PD). This study reports tests for PD in HOP tests on 9 infertile patients and 11 normal controls. The data, presented in detail, show the expected PD in the control group. In contrast, 3 patients had HOP tests with definite non-PDs, whereas 5 patients have clearcut PDs. The cause of non-PD is unknown, but could result from differences among motile sperm in penetrating ability.