We have studied the cellular and molecular basis of eight cases of infant null acute lymphoblastic leukemia (ALL). All eight patients were under 9 months of age and presented with leukocyte counts in excess of 60 X 10(9)/L, organomegaly, and in two cases CNS infiltration. Although seven cases were morphologically classified as ALL, one patient had both lymphoid and myeloid features. Phenotypic analysis of leukemic blasts from all patients showed a typical null ALL pattern, ie, CD10 (common ALL antigen)-negative, strongly HLA-DR-positive, and CD19 (B4)-positive. The presence of terminal deoxynucleotidyl transferase (TdT) at presentation was positive in six patients' cells and negative in two. Two patients also expressed the myeloid-associated markers CD33 (MY9) and CD15 (TG1), and coexpression of CD19 and CD33 was confirmed in these two by using dual marker flow cytometry (fluorescence-activated cell sorting). Electron microscopic examination of the same two patients' cells showed the presence of monocytoid blasts that labeled with the pan-B cell antibody B4 (CD19). Short-term culture of one of these patients cells in the presence of phorbol ester resulted in the majority of the cells exhibiting myeloid markers, strong nonspecific esterase positivity, and phagocytic properties. Cytogenetic analysis showed the common feature in 7 of 8 cases to be a break in band 11q23. Molecular analysis of DNA from the blast cells of all eight patients showed rearrangement of the immunoglobulin heavy-chain genes in all cases without, however, any evidence of kappa light-chain rearrangement. T cell receptor genes were present in the germline configuration in all cases. Rearrangements of the c-ets 1 oncogene, which maps to band 11q23, were not detected, thus providing no evidence for involvement of this oncogene in the common disease process. Our data indicate that although infant null ALL may present as a heterogeneous disease the similarity of many features between cases suggests a common derivation from a precursor cell sharing phenotypic and genotypic features of both B and myeloid progenitor cells.