Serum protein electrophoresis was performed in 68% of 1,682 consecutive patients with lymphoma. Of 400 patients with chronic lymphocytic leukemia and lymphocytic lymphoma, 2.3% had an IgG peak, a frequency significantly higher than that found in normal individuals of comparable age. IgM peaks occurred in 4.5% of patients with lymphomas characterized by diffuse histologic infiltration of lymph nodes. This frequency was about 100 times greater than predicted and indicated that most IgM peaks detected by electrophoresis are produced by lymphomas. Clinical responses to therapy were associated with reductions of tumor mass usually ranging from 10% to 50% of the pretreatment level. A much greater magnitude of tumor reduction is needed to improve remission duration for patients with chronic lymphocytic leukemia and lymphocytic lymphoma.