We have studied the thrombocytopenia of lymphoproliferative disorders using a measurement of membrane-bound IgG by an antiglobulin consumption assay. Nine patients with chronic lymphocytic leukemia (CLL) and thrombocytopenia had increased membrane-bound IgG. Two patients with non-Hodgkins lymphoma and 1 patient with Hodgkins disease also had thrombocytopenia and increased membrane-bound IgG. Five of the patients with CLL had positive direct antiglobulin (Coombs) tests on red cells; of these, 3 patients had hemolytic anemia. In eight of the 9 patients with CLL, thrombocytopenia, and increased platelet-bound-IgG, the platelet count increased with the administration of prednisone or an alkylating agent, with splenectomy, or with a combination of these.