Sperm, during transit through the epididymis, acquire the capacity to fertilize ova as assessed via in vitro fertilization assays. While alterations in many spermatozoal features may be important, it is clear from a variety of studies using many species that modification of membranes represents one essential change. The many reports of changes in the ability of sperm to undergo the acrosome reaction and ova binding serves as one example. This laboratory has made major contributions to the quantification of changes in membrane features by introducing new physical methods to assess membrane status and completing the first analysis of the lipid components of the plasma membrane overlying the acrosome. These published data, plus preliminary data, are combined with newly introduced concepts regarding the specific relationship of lipid composition on membrane stability to yield a unique hypothesis regarding the molecular events that are necessary for sperm to acquire the capacity, during epididymal maturation and ejaculation, to undergo the acrosome reaction. The hypothesis presented for test is as follows: Lipid modification is essential to the acquisition of the capacity to undergo the acrosome reaction. Sperm are viewed as serving in a passive role with regard to membrane modifications, in that alteration of their surfaces results from interaction with specific lipoprotein donors and lipid exchange proteins found in their surrounding fluids. The specificity of the transfer is such that the our measured changes of removal of one mole of phosphatidylethanolamine and insertion of one mole of phosphatidylcholine plus three moles of cholesterol in the plasma membrane overlying the acrosome results in a "stressed" inner half of the plasma membrane bilayer. Release of this stress, by unknown factors after deposition in the female tract, results in the acrosome reaction. To test this hypothesis, five projects are outlined. The first will compare multiple assays of the in vitro induced acrosome reaction. The second and third will yield data sufficient to characterize the lipid exchange proteins and lipoproteins of male reproductive tract fluids. The fourth project will establish methods necessary to systematically alter sperm membrane composition in vitro (to mimic in situ changes) while the final project will provide a summary test of the effect of known changes in lipid composition on the capacity of immature epididymal sperm to undergo an in vitro induced acrosome reaction.