Specific binding of tritium-labeled platelet-activating factor (PAF) and a nonmetabolizable bioactive analog of PAF, 1-O-alkyl-2-N-methylcarbamyl-sn-glyceryl-3-phosphorylcholine, to human platelet membranes was found to be potentiated by wheat germ agglutinin (WGA) and erythroagglutinin. As demonstrated in Scatchard plots, the potentiation effect is due to an increase in the maximal number of receptor sites, with no alteration in the equilibrium dissociation constant. The WGA-potentiated specific binding can be specifically inhibited by N-acetylglucosamine, shows identical affinity for PAF agonists and a receptor antagonist, L-659,989, and has an identical Na+ inhibition pattern to non-treated membranes in the absence of WGA. The WGA-induced potentiation is preferential in the plasma membrane-enriched fraction. The maximal number of receptor sites increases in membranes pretreated with neuraminidase and beta-N-acetylglucosaminidase. Therefore, WGA may bind to an endogenous PAF receptor modulator, which then either dissociates from or associates with the PAF receptor and regulates the receptor conformation. The membrane fraction enriched with intracellular membranes is also enriched with PAF receptors. WGA was also found to increase the maximal aggregation of rabbit and human platelets induced by PAF and to induce the synthesis of PAF, which preceded aggregation in human platelets. An intracellular PAF receptor may also exist, and it could modulate the function of PAF retained inside of the stimulated cells.