Monoclonal antibodies reacting with external domains of plasma membrane proteins of sea urchin spermatozoa cause the reversible aggregation of sperm. The rate of disaggregation is temperature dependent indicating the involvement of membrane fluidity. Immunofluorescence shows that disaggregation is temporally correlated with the movement of surface-bound antibody into large aggregates which are then shed from the cell surface. Electrophoretic analysis of the shed antigens shows that they resemble the total complement of proteins of isolated sperm membranes. The translocation and shedding of surface-bound ligands may be a common property of eukaryotic flagellar membranes.