The adhesion-promoting activity of TSP has been examined in four systems. Both the amino-terminal heparin-binding domain and the large globular carboxy-terminal domains have been implicated in the agglutination of erythrocytes and fixed-activated platelets by TSP. Sulfatides specifically bind TSP and may serve as a membrane receptor for the heparin-binding domain of TSP. The large globular domains of TSP have been implicated in secretion-dependent platelet aggregation. Activation-dependent adhesion of platelets to TSP substrates does not appear to be mediated solely by direct interaction of TSP with platelet membrane components, but appears to be mediated in large part by intervening bridging molecules, such as fibronectin or fibrinogen. Thus, different regions of the complex TSP molecule are of varying importance in the different systems. No single simple system appears adequate for exploration of the adhesion-promoting activity of TSP.