Blast cells from 6 of 50 patients with acute lymphoblastic leukemia (ALL) displayed intracytoplasmic mu chains in the absence of detectable light chains and surface immunoglobulins. These cells also expressed lalike and common ALL antigens. Terminal deoxynucleotidyltransferase was detectable in 2 of 5 cases tested. These blast cells are probably related to early B-cell precursors (pre-B cells). In 4 of 6 cases the disease had a tumoral presentation; the prognostic significance of this new subgroup, which accounts for 20% of patients with non-T non-B ALL, remains to be established.