The patient described here had a nodular, poorly differentiated lymphocytic lymphoma associated with a serum monoclonal protein, IgG lambda. Following a three year period of radiation-induced clinical remission she developed generalized diffuse histiocytic lymphoma. Direct immunoperoxidase staining of the tissue sections demonstrated that the neoplastic cells of each biopsy only contained IgG lambda immunoglobulin, identical to the serum monoclonal protein. This is presumptive evidence that these two histopathologically distinctive malignant lymphomas, occurring consecutively in the same patient, were responsible for the synthesis and secretion of the same serum M component. This strongly suggests that both lymphoid neoplasms arose from the same malignant clone. The results 1) confirm the light microscopic observation that nodular lymphocytic lymphoma may progress to diffuse histiocytic lymphoma and 2) offer further evidence that histiocytic lymphomas arising in patients with previous B cell malignancies are most probably related to the original B cell proliferation and do not represent the emergence of a second, separate malignant clone.