Sperm surface glycoproteins may be involved in sperm-zona pellucida recognition. Some of these coating proteins are of seminal plasma origin and their expression may change in the process of capacitation and acrosome reaction. Sperm specific monoclonal antibodies (mAb) define the presence and role of sperm membrane associated proteins. We have isolated a monoclonal antibody (SEM-12) specific for human sperm that shows, by indirect immunofluorescence, a discontinuous distribution of the antigen on the head and tail surfaces of non capacitated sperm. This antigen is also present in human seminal plasma as detected by ELISA. The antigen is detectable in sperm of goat, ram, and mouse. Two proteins in the range of 80-84 kDa have been isolated by affinity chromatography with SEM-12 mAb. The same result is obtained by immunoprecipitation. This antibody inhibits sperm motility and acrosome reaction (spontaneous and A23187 ionophore induced.