The surface proteins of intact rat, ram, boar and bull spermatozoa from caput and cauda epididymidis were radioiodinated and analysed by SDS-polyacrylamide gel electrophoresis. The maturational changes in sperm surface protein patterns of the different species were compared to elucidate any possible common features or differences in the surface protein reorganization mechanism of mammalian spermatozoa during epididymal passage. New labelled proteins appeared on the surfaces of the rat and ram spermatozoa, whereas bull spermatozoa lost some surface protein components during maturation. The surface proteins of boar spermatozoa could be only weakly labelled by radioiodination and no distinct maturational change in the radioactivity patterns of labelled boar spermatozoa could be shown. The analysis of radioiodinated ram spermatozoa also revealed an obvious transformation of the labelled membrane lipids during epididymal transit. The conclusion is that the surface proteins of mammalian spermatozoa undergo some changes during epididymal maturation but there is no uniform mechanism in these changes common to all mammalian species.