High-resolution two-dimensional polyacrylamide gel electrophoresis analyses have indicated that several major boar sperm plasma membrane polypeptides (PMPs) increased in concentration during maturation in the epididymis. To investigate this further, monoclonal antibodies (MAbs) to two of these PMPs/glycoproteins referenced as 4.85 and 5.0 and polyclonal antisera (PCA) raised against PMP 5.0 were used to quantify these changes. ELISA assays for both PMPs, using solubilized plasma membrane (PM) and intact PM as antigens, indicated that both PMPs were present in greater concentration in cauda PM than in caput PM. ELISA assay using PCA against 5.0 and whole sperm from the cauda and caput epididymis also showed an increase in the concentration of this protein during transit through the epididymis. Indirect FITC fluorescence microscopy showed that MAbs reacted specifically with the luminal aspect of secretory cells of the corpus epididymis and, with greater intensity, with secretory cells of the cauda segment. When MAb tags were used the fluorescence technique indicated that PMP 5.0 was restricted to a narrow area of PM overlying the principal segment of the head of caput sperm but fluorescence extended further into the principal segment and into the flagellum of cauda PM. Since MAb binding sites appear to be partially masked, PCA were also used to localize antigenic sites. PCA to PMP 5.0 showed antigen localization in both head and flagellar PM of cauda and caput sperm. The fluorescence of cauda sperm PM, however, was significantly more intense than that of caput sperm PM. MAb to PMP 4.85 also bound to antigen more extensively on cauda sperm. The sum of these analyses suggests that major sperm PMPs are secreted by the epididymis and absorbed by sperm. The magnitude of surface protein changes occurring during epididymal transit in boar sperm appears to be greater than heretofore recognized.