A case involving a 67 year old man with an immunocytoma initially presenting as Waldenström's macroglobulinemia which transformed into a high grade malignant lymphoma of immunoblastic type after treatment with cyclophosphamide and corticosteroids over 5 months is presented. IgM (kappa) present in the serum in the phase of immunocytoma was demonstrated also on the less differentiated cells of the high grade lymphoma. The permanent production of the same immunoglobulin molecule suggests that both morphologically different malignancies derived from one B cell clone.