Using an improved electroimmunofixation technique that combines the sensitivity of high resolution agarose gel electrophoresis with the specificity of immunoprecipitation, we have demonstrated monoclonal immunoglobulin bands in the serum of patients with undifferentiated lymphomas of Burkitt and non-Burkitt types. Monoclonal bands were detected in the serum of 12 of 21 patients with extensive tumor, and 1 of 10 patients with minimal tumor. All of the bands were identified as IgM of a single light chain class. Such bands were not detected in the serum of patients with lymphoblastic lymphoma (7) or African Burkitt's lymphoma (6). There was disappearance of the bands after therapy and reappearance at relapse. These findings, coupled with previously reported in vitro information, indicate that undifferentiated lymphoma cells secrete immunoglobulin of IgM isotype. Therefore, such monoclonal bands may be of potential value as tumor markers.