B cell chronic lymphocytic leukemia (CLL) is a malignancy of the CD5+ B cells. Prior studies indicated that CLL B cells generally express immunoglobulin (Ig) VH and VL genes with little or no somatic mutations. However, a recent report indicated that VH251, one of three VH genes belonging to the VH5 subgroup (e.g., VH251, VH32, and VH15), not only is frequently rearranged in this disease, but also has extensive and selective mutations when expressed by CLL B cells. The extent and nature of these mutations contrasts markedly from the low level of mutations noted in VH5 genes used by normal B cells or other Ig V genes found expressed in CLL. To determine whether this difference reflects a unique property of VH251 or a previously unrecognized subgroup of CLL, we examined for VH5 Ig gene rearrangements in leukemia cells from 68 patients that satisfied clinical and diagnostic criteria for CD5+ B cell CLL. Southern blot hybridization studies with probes for VH251 and the JH locus revealed that only 7 (10%) of the 68 monoclonal CLL cell populations had undergone Ig gene rearrangement involving VH5 genes. Two (3%) were found to have functionally rearranged VH5 genes that shared > or = 98% sequence homology with 5-2R1, a VH251 gene isolated from a pre-B cell acute lymphocytic leukemia. The other five CLL (7%) had functionally rearranged VH5 genes that each shared > or = 99% nucleic acid sequence homology with a germline VH32 isolated from human sperm DNA. These data indicate that VH251 or VH32 also may be expressed by CD5+ CLL B cells with little or no somatic mutation. These findings contrast with a recently published study on VH5 gene expression in B CLL and contest the hypothesis that extensive somatic mutation is a common property of the VH5 genes used in this disease. Further work to define the clinical and/or phenotypic characteristics of patients with leukemia cells that express mutated versus nonmutated Ig V genes may reveal subsets of CLL that possibly differ in their cytogenesis, etiopathogenesis, and/or clinical behavior.