We analyzed 50 B cell lymphoma samples by Southern blot analysis, using the bcl-1 and heavy chain immunoglobulin (JH) probes with two or more restriction endonucleases. All samples showed JH rearrangement, and three samples (two diffuse small lymphocytic lymphomas and one diffuse large cell lymphoma probably transformed from a diffuse small lymphocytic lymphoma) demonstrated rearranged bcl-1 sequences. The three samples showed the t(11;14)(q13;q32) chromosome translocation, and all three contained rearranged JH fragments that comigrated with the rearranged bcl-1 fragment. The breakpoint of the translocation occurred within a 1.6-kb region on chromosome 11 in the three cases. Two of the three patients had primary refractory disease. Two of the three patients had gastrointestinal involvement. Bcl-1 rearrangement may identify an unusual subset of patients with primary refractory disease with gastrointestinal involvement. It may also describe a unique subset of large cell lymphoma patients transformed from diffuse small cell histology.