The authors report a case of suramin-induced immune-mediated thrombocytopenia occurring in a patient on a Phase II protocol for treatment of metastatic renal cell carcinoma. On withdrawal of suramin, the acute and severe thrombocytopenia did not respond initially to plasmapheresis; subsequently, it resolved promptly and dramatically during the administration of intravenous gamma globulin, also coinciding with a decline in suramin blood levels. Results of immunologic and coagulation studies are reported and clinical implications discussed.