Thirty-five infertile men were divided into two groups: those with normal spermograms (15 cases) and those with oligospermia (20 cases). Each semen sample was examined by its penetration into zona-free hamster ova and by an improved Coulter counter apparatus for measuring the volume of spermatozoal head. Both groups were further divided into two subgroups: those with normal (above 15%) and those with abnormal penetration assay (0-10%). The relationship between sperm modal size and sperm cell penetration into hamster ova in both groups--the oligospermic and the normospermic--was calculated and was found to be nonsignificant (correlation index: -0.154). It appears that the inability of some human sperm cells to penetrate the zona-free hamster ova cannot be attributed to the spermatozoal size.