Monoclonal antibodies were prepared to guinea pig platelets and selected for their ability to inhibit ADP-induced platelet aggregation and ristocetin induced, Ca++-independent platelet agglutination. One antibody, PG-2, produced strong inhibition of aggregation induced by ADP, thrombin, collagen and arachidonic acid, while not inhibiting ristocetin-induced agglutination. A second antibody, PG-1, blocked ristocetin-induced agglutination, but did not inhibit aggregation induced by the previous agents. PG-2 blocked ADP-induced 125I-fibrinogen binding to washed guinea pig platelets by approximately 50%, but did not inhibit ristocetin-induced binding of 125I-vWF. Conversely, PG-1 selectively inhibited ristocetin-induced 125I-vWF binding, with the degree of inhibition inversely related to the ristocetin concentration. These studies suggest that in guinea pig platelets, fibrinogen and von Willebrand factor binding to different membrane sites are responsible for the aggregation response of stimulated platelets and the ristocetin-induced agglutination response respectively. These antibodies offer significant promise for the further development of a guinea pig animal model for studying platelet and megakaryocyte function.