A variety of sperm surface components have been suggested to mediate gamete recognition by binding to glycoside ligands on the egg coat glycoprotein ZP3. The function of each of these candidate receptors is based upon varying degrees of circumstantial and direct evidence; however, the effects on fertilization of targeted mutations in any of these candidate receptors have not yet been reported. In this paper, we describe the effects of targeted mutations in β1,4-galactosyltransferase, the best studied of the candidate receptors for ZP3. Surprisingly, galactosyl-transferase-null (gt^−/−) males are fertile; however, sperm from gt^−/− males bind less radiolabeled ZP3 than wild-type sperm, and are unable to undergo the acrosome reaction in response to either ZP3 or anti-galactosyltransferase anti-bodies, as do wild-type sperm. In contrast, gt^−/− sperm undergo the acrosome reaction normally in response to calcium ionophore, which bypasses the requirement for ZP3 binding. The inability of gt^−/− sperm to undergo a ZP3-induced acrosome reaction renders them physiologically inferior to wild-type sperm, as assayed by their relative inability to penetrate the egg coat and fertilize the oocyte in vitro. Thus, although ZP3 binding and subsequent induction of the acrosome reaction are dispensable for fer-tilization, they impart a physiological advantage to the fer-tilizing sperm. A second strain of mice was created that is characterized by a loss of of the long galactosyltransferase isoform responsible for ZP3-dependent signal transduc-tion, but which maintains normal levels of Golgi galacto-sylation. Sperm from these mice show that the defective sperm-egg interactions in gt^−/− mice are due directly to a loss of the long galactosyltransferase isoform from the sperm surface and are independent of the state of intracel-lular galactosylation during spermatogenesis.