We investigated leukemic cells with multiple immunoglobulin heavy chain (IgH) gene rearrangements from nine B-precursor cell acute lymphoblastic leukemia (ALL) patients and three chronic myelocytic leukemia lymphoid crisis (CML.Ly-BC) patients in order to determine detailed recombination patterns of the variable (V), diversity (D), and joining (J) region genes. Southern blot study, using DNA probes for DQ52 and 5'D region genes, was useful to distinguish VDJ recombination from DJ recombination at the level of each allele. Leukemic cells from seven out of eight CD10-positive ALL patients showed biallelic VDJ recombinations. Rearrangements of Ig kappa genes were found in only one case. Leukemic cells from all of the CML.Ly-BC patients had a DJ/(V)DJ IgH genotype. These findings suggest that the multiple IgH gene rearrangements in B-precursor cell ALL occurred as a consequence of continuing V-(V)DJ rearrangements after neoplastic transformation, and were closely related to the stage of bone marrow B-precursor cell differentiation. Multiple IgH gene rearrangements in CML.Ly-BC might take place earlier in the process of IgH gene rearrangements than is the case in B-precursor cell ALL. In this sense, the genotypic oligoclonality observed in ALL and CML.Ly-BC should be regarded not as 'true', but as 'pseudo' oligoclonal leukemia.