To characterize the platelet receptor sites and the platelet metabolic pathways involved in tumor-cell-induced platelet aggregation, we have used a homologous system consisting of human platelets and 2 tumor cell lines of human origin, which activate platelets through different mechanisms. Preincubation of platelets with an MAb against platelet glycoprotein Ib partially blocked tumor-cell-induced platelet aggregation, and preincubation of platelets with an MAb against the glycoprotein complex GPIIb/IlIa totally blocked the aggregation induced by the 2 tumor-cell lines. No inhibitory effect was found when platelets were treated with PAF-receptor antagonists or with specific peptides which block the platelet sites involved in bacterially induced platelet aggregation. Compounds which raised intra-platelet CAMP levels inhibited tumor-cell-induced platelet aggregation in a dose-related manner. Inhibition of cyclo-oxygenase by aspirin which blocked TxB2 formation by platelets did not inhibit platelet aggregation induced by tumor cells whereas the BW755 compound which inhibits cyclo-and lipoxygenase blocked platelet aggregation. These results demonstrate that tumor-cell-induced platelet aggregation is a glycoprotein-dependent and a lipoxygenase-associated phenomenon.