The capacity of glass wool-filtered spermatozoa to penetrate zona-free hamster oocytes was studied. As compared to prefiltered sperm samples, oocyte penetration was significantly increased. A significant increase in the penetration rate for the filtered sperm population was noted even after the sperm motility in the filtrate was adjusted with medium equal to that of the prefiltered sample. However, no significant differences in oocyte penetration were seen between the prefiltered and the filtered sperm population when the filtered sperm samples were diluted with nonviable spermatozoa. These results show that glass wool filtration yields a sperm population with a greater penetrating capacity. It was concluded that motility alone could not account for the improved penetrability and that the removal of nonviable spermatozoa may at least, in part, be responsible for this effect.