A 67-year-old man with Waldenström's macroglobulinemia had a relapse of chronic idiopathic thrombocytopenic purpura (ITP), which had been refractory to corticosteroids, splenectomy, vinca alkaloids, and high-dose intravenous gamma-globulin therapy. A biclonal gammopathy (IgM kappa and IgG lambda) was detected in his serum and was likely responsible for his refractory thrombocytopenia. He was treated with chlorambucil and prednisone. Plasmapheresis was effective in temporarily maintaining platelet counts and in decreasing morbidity until immunosuppression was completely effective against the production of the monoclonal protein. The previously reported experiences with use of plasmapheresis in patients with chronic ITP are discussed. Plasmapheresis may be of value in the treatment of selected patients with severe ITP and monoclonal gammopathy.