The concept of lineage fidelity in acute leukemia has recently been challenged by the finding of rearrangements of the immunoglobulin heavy chain genes in a leukemic cell line and in a small number of sporadic cases of acute nonlymphocytic leukemia with a monocytic phenotype. We therefore screened leukemic blood or bone marrow samples of 33 adult patients with acute nonlymphocytic leukemia of FAB types M4 (23 patients) and M5 (10 patients); 28 were obtained at diagnosis and 5 at relapse. All cases were well characterized pathologically and histochemically. Cytogenetic analysis performed in each case demonstrated karyotypes that were representative of those generally seen in these types of leukemia, with a clonal abnormality present in all except 9 of 32 patients who were successfully studied. DNA prepared from each sample was digested with the restriction enzyme BamH1 and analyzed by Southern blot hybridization to probes for the JH region of the immunoglobulin heavy chain. All 33 cases had DNA retained in the germline configuration with no evidence of rearrangement. This finding supports the concept of lineage fidelity, and suggests that true interlineage infidelity, myeloid to lymphoid, is a rare occurrence in adult acute nonlymphocytic leukemia.