As part of an investigation of the cellular origins of undifferentiated lymphomas of the Burkitt's and non-Burkitt's types, we have examined immunoglobulin secretion by cell lines and biopsy samples from these tumors and have compared it with other lymphoblastoid cell lines. The majority of American lymphoma cell lines and a smaller fraction of African lymphoma cell lines secreted monoclonal IgM. No other Ig classes were secreted. Ig secretion by the tumor samples was also found to be exclusively IgM. The secreted IgM was polymeric and associated with J chain. Secretion of free light chain by most of the tumor cell lines was observed. Quantitation by immunoassay demonstrated that the American lymphoma cell lines secreted significantly more IgM than the African lymphoma cell lines. We have proposed that the lymphoma cells represent an early stage of B cell differentiation, during which antigen-independent secretion occurs.