Twenty-five patients with B-cell chronic lymphocytic leukemia (B-CLL) were investigated to correlate the immunological phenotype with the description of the Ig gene rearrangements of the B-cell clone. All patients were positive for the CD19 antigen and one pan B-antigen, markers of late cells (CD20, CD37 or Y2955). Twenty-four of the 25 patients tested expressed monoclonal cell surface immunoglobulin (SIg). The CD5 antigen was present in 21 of the 25 tested patients. Immunoglobulin gene rearrangements were detected by hybridization of the BamHI, EcoRI, BgIII, and HindIII digested genomic DNAs to the IGHJ, IGKC, IGLC, and IGLJ2 probes. Twenty-four of 25 patients had two rearranged IGH loci. The IGKC rearrangements were observed in 20 patients. In four patients, the IGK loci were deleted on both chromosomes. One patient without SIg displayed a germline pattern. All six patients with lambda producing B-CLL showed a lambda gene rearranged band, although the use of IGL polymorphism to investigate IGL rearrangements must be noted. These clonal rearrangements of IGL genes, together with the detection of either the kappa or lambda light chain of SIg, confirm that patients with B-CLL meet the developmental scheme of ordered light chain gene rearrangements.