Selectins are a family of adhesive molecules, involved in the interactions between leukocytes and endothelium and in platelet adhesion. P-selectin, one of the members of this family, is stored in alpha-granules and dense granules of platelets as well as in Weibel-Palade bodies of endothelial cells, and it is rapidly redistributed to the cell surface after activation. It recognizes carbohydrate structures as ligands, in particular sialyl-Lewis(x), which is part of the CD15 antigen. In this work we studied P-selectin expression on gametes. While zona-free human and hamster oocytes did not react with a monoclonal antibody directed against P-selectin, oocytes from both species displayed a reactivity with this antibody following their contact with human spermatozoa, as demonstrated both by covasphere binding and indirect immunofluorescence. Artificial activation of zona-intact human oocytes by means of the calcium lonophore A23187 induced the expression on the oolemma of a moiety reacting with anti-P-selectin antibody as well. P-selectin also appeared to be expressed on the sperm surface following the acrosome reaction, as demonstrated by a flow cytometric study of reactivity of spermatozoa with the anti-P-selectin antibody, using the expression of CD46 as a marker of the acrosome reaction. The localization of the P-selectin moiety on the equatorial region of the plasma membrane of acrosome reacted spermatozoa was confirmed by transmission electron microscopy using immunogold labelling. We suggest that P-selectin might be involved in gamete interactions.