Studies on mouse sperm-egg binding and fertilization have been suggested to involve the interaction of sperm-associated β1-4 galactosyltransferase with egg zone pellucida glycoproteins. A population of human males, whose sperm demonstrated an inability to penetrate ovulated zona pellucida-free hamster eggs in vitro, were examined for the level of activity of β1-4 galactosyltransferase. The level of enzyme activity was found to be reduced in human sperm isolated from this group of individuals compared with a known hamster penetration-positive group. Analysis of the deoxyribonucleic acid from these individuals by Southern hybridization with a putative human complementary deoxyribonucleic acid clone to β1-4 galactosyltransferase identified a unique allele lacking 0.8 and 0.4 kb restriction fragments on digestion with the endonuclease Taq I. These results represent the first evidence to suggest that mutations could be associated with the human gene for galactosyltransferase. Our data help to clarify one of the possible molecular mechanisms responsible for sperm-egg binding/penetration interactions. © 1989.