The t(11;14)(q13;q32) is a recurring translocation associated with some chronic B-cell lymphocytic malignancies; the putative protooncogene BCL1, located at the chromosome band 11q13, can be involved during the translocation process. In order to determine if BCL1 rearrangement is associated with a particular subtype of lymphoma, we analysed 131 B-cell non-Hodgkin's lymphoma samples by Southern blot analysis, using a BCL1 probe. The BCL1 locus was rearranged in 9 out of 25 (36%) cases of intermediate lymphocytic cell lymphomas (ILL), in 1 out of 8 cases of diffuse small cleaved cell lymphoma, in 1 out of 12 cases of diffuse mixed cell lymphoma, and in 1 out of 21 cases of diffuse large cell lymphoma. In contrast, BCL1 was never found rearranged in any of the 46 follicular lymphomas analysed. The BCL2 gene was in germ-line configuration in all ILL. Sequential hybridization of Southern blots with JH, C mu, and BCLI probes identified comigrating fragments in only one case of ILL, which suggests that, in all the other cases, either the rearrangement of BCL1 did not result from a t(11;14) translocation or the break on chromosome 14 occurred outside the JH or C mu regions. These results indicate that rearrangement of the BCL1 locus may be closely associated with ILL and could be considered as a genotypic marker of this lymphoma subtype.