Ig heavy chain (IgH) J (JH) region organization in 83 human acute leukemias was studied. In 31 of the 35 precursor B acute lymphocytic leukemia (ALL) the rearrangements of the IgH gene involved sites between D regions 5' to DQ52 and JH. The study of the IgH gene organization in 43 T lineage ALL showed nine cases with the rearrangement at the IgJH region, one of which involved the D region 5' to DQ52. When analyzed with another restriction enzyme (BglII), three of the remaining eight showed rearrangement between DQ52 and JH. In the remaining five samples rearrangement of the IgJH region was not shown, but the restriction fragment length polymorphism (RFLP) between MspI sites in 5'DQ52 loci was observed. Thus the RFLP in 5'DQ52 locus was identified and was distinguished from the recombination between DQ52 and JH in non-B lineage leukemias.