The effect of morphology on the ability of human spermatozoa to penetrate zona-free hamster oocytes was investigated using fractions of individual semen samples enriched for morphologically normal or abnormal spermatozoa, after separation by buoyant density centrifugation on self-generating Percoll gradients. Using this separation technique, the percentage of spermatozoa of normal morphology is greater in the lowermost 1 ml of the Percoll column (layer 1) than in the fraction recovered from higher up the column. Thus it was possible to compare two populations of spermatozoa from each semen sample in their ability to penetrate hamster oocytes. After normalizing the concentration of motile spermatozoa the mean hamster oocyte penetration rate was significantly higher with spermatozoa recovered from layer 1 (39 +/- 4%) compared with those recovered from layer 7 (21 +/- 4%), which suggests that morphologically abnormal spermatozoa do not have the same fertilizing capacity as those with normal morphology.