In 35 of 191 patients with acute lymphocytic leukemia (ALL) malignant cells were similar in phenotype to B-lymphocyte precursors. Both these patients' lymphoblasts and normal pre-B-cells contain cytoplasmic immunoglobulin (Ig) μ heavy chains, but have no surface Ig. In patients with pre-B leukemias, lymphoblasts containing cytoplasmic μ chains alone were often accompanied by cells of identical morphology that expressed no Ig and less frequently by lymphoblasts bearing scant amounts of surface μ. This spectrum of cellular Ig expression suggests that 'null', pre-B, and intermediate pre-B/B ALLs represent closely related malignancies with complete or partial arrests at different stages of maturation. When pre-B, B, T, and 'null' cell categories of ALL were compared for 22 different clinical and laboratory features, including remission rate and short-term remission duration, no statistical differences were observed between the pre-B and 'null' groups. These early results suggest that pre-B-cell leukemias represent a relatively good prognostic subclass of ALL, do not require more intensive treatment than that proven to be effective for 'null' cell ALL, and should be distinguished from the less common, but more clinically aggressive, B-cell subclass of ALL. Longer follow-up will be required to confirm these preliminary conclusions.