V(D)J recombination and somatic hypermutations are developmentally regulated during B-cell differentiation; therefore, DNA analysis of the Ig gene delineates the cellular origin of B-cell neoplasms. We analyzed the third complementarity-determining region and adjacent regions of the Ig heavy-chain gene of tumor cells from 7 patients with Waldenström's macroglobulinemia (WM) and from 10 patients with B-cell chronic lymphocytic leukemia (CLL), 2 of whom progressed to high-grade non-Hodgkin's lymphoma (NHL), ie, Richter's syndrome (RS). There were no intraclonal variations resulting from VH replacements or ongoing somatic mutations in both WM and CLL. We found replacement mutations in the D and/or JH segments in all patients with WM and in 4 of the 10 patients with CLL, including the 2 RS patients. Replacement mutations were clustered in codon 102 of the JH segment. Preferential utilization of the JH4 gene was found in WM (5 of 7 [71.4%]) and in CLL (7 of 10 [70.0%]), and DXP family genes in CLL (5 of 10 [50.0%]). In conclusion, WM and CLL with RS are generated under the influence of antigenic stimulation and selection. However, the majority of CLL may arise from a distinct subpopulation that has the restricted repertoire of nonmutated Ig genes.