Recently the minor B cell subpopulation that expresses the CD5 (Leu-1) antigen has been implicated as a source of IgM autoantibodies. Chronic lymphocytic leukemia (CLL), the most common leukemia in humans, represents a malignancy of small B lymphocytes that also express the CD5 antigen. However, little is known concerning the antibody variable region genes (V genes) that are used by these malignant CD5 B cells. We have found that a relatively high frequency of CLL patients have leukemic B cells with surface immunoglobulin (sIg) recognized by 17.109, a murine mAb specific for a kappa light chain associated crossreactive idiotype (CRI) associated with rheumatoid factor and other IgM autoantibodies. Flow cytometric analyses revealed that the relative expression of the 17.109-CRI by circulating leukemic B cells was directly proportional to the levels of sIg kappa light chain, indicating that there exists stable idiotype expression in the leukemic population. To examine this at the molecular level, the nucleic acid sequences encoding the Ig kappa light chains of two unrelated patients with CLL bearing sIg with the 17.109-CRI were determined. Analyses of multiple independent kappa light chain cDNA clones did not reveal any evidence for sequence heterogeneity in the CLL cell population. Furthermore, the nucleic acid sequences expressed by the leukemic cells of these two patients were identical or very homologous to a germline V kappa gene isolated from placental DNA, designated Humkv 325, or "V kappa RF" because of its association with IgM autoantibodies. This study suggests; (a) that the malignant CD5+ B lymphocytes in CLL use the same V kappa gene that has been highly associated with IgM autoantibodies and (b) that the expression of V genes is stable in CLL, in contrast to other B cell malignancies examined to date. We propose that many CLL cases represent malignancies of autoreactive CD5 B cells that use a restricted set of conserved V genes. This property may render CLL particularly amenable to immunotherapy with antiidiotypic antibodies.