Four childhood acute leukemias with morphological, cytochemical and immunological characteristics correlating to precursor B lymphocyte and with germline configuration of the immunoglobulin heavy chain joining region were studied for the organization of the C mu segment. Two of the four lymphoblast samples retained the germline configuration of both JH and C mu regions. The other two samples showed delection of the entire JH region resulting in the rearrangement of the C mu region. In contrast to patients with C mu rearrangement, patients with C mu in germline form were not able to achieve complete remission after induction therapy. Study of the C mu region in patients with JH segment in germline configuration could separate subgroups in childhood precursor B acute lymphoblastic leukemia with different prognoses.