Rearrangements of immunoglobulin (Ig) heavy chain (JH) and T cell receptor beta chain (T beta) genes of lymph node cells from 28 patients with malignant lymphoma were studied. Fourteen of 28 patients were diagnosed as undetermined cell lymphomas, since a marked predominance of cells expressing a single light chain of Ig or T cell associated antigens was not demonstrated by phenotypic analysis. Of these patients, JH gene rearrangements were observed in 8 patients, whereas T beta gene rearrangements were detected in 9 patients. Although bigenotypes were seen and cell lineage was not determined in three cases, these results indicate that demonstration of Ig and T beta gene rearrangements is a powerful tool to determine cell lineage and clonality of lymphoma cells that do not express definitive surface phenotypes. Interestingly, 12 of 14 lymphoma cells whose lineage had been determined by phenotypic analysis showed rearrangements of two alleles; however, only single allelic gene rearrangement was observed in 11 of 14 lymph node cells from phenotypically undetermined cell lymphoma.