The ability of tumor cells shed into the circulation to cause adhesion and aggregation of blood platelets may be involved in successful metastasis of primary tumors. Rhabdosarcoma is a rare, early metastasizing tumor previously uncharacterized for ability to alter platelet function. It was found that human rhabdosarcoma cells (American Type Culture Collection) dose dependently induce biphasic aggregation of human blood platelets in heparinized platelet-rich plasma; aggregation responses could also be elicited in citrated plasma. Aggregation caused by rhabdosarcoma can be inhibited by apyrase treatment of either rhabdosarcoma or platelets, and by pretreatment of platelets with prostacyclin, cilostamide, inhibitors of thromboxane A2 production, or TMB-8; only apyrase and prostacyclin inhibited both phases of aggregation. Tumor cell supernatant contained only enough ADP to cause a negligible, reversible aggregation response. Hirudin, verapamil, and triazolam do not inhibit rhabdosarcoma-induced aggregation. Aggregation of platelets by rhabdosarcoma cells thus appears to involve ADP, from tumor cells and/or platelets, and platelet calcium mobilization and thromboxane A2 synthesis and release.