More than 90% of both unfractionated and blast-enriched T cell-depleted mononuclear cells (MNC) from a patient with a morphologically and phenotypically unclassified acute leukemia expressed interleukin 2 receptors (IL2-R), whereas 50% unfractionated MNC displayed monocyte/myeloid- (My7, My9, and OKM1) and B cell-restricted (B4) antigens. Two-color fluorescence studies showed that 80% of the My9+ cells expressed the B4 antigen and 63% of the B4+ cells were IL2-R+. Cell incubation with phorbol myristate acetate (PMA) increased the expression of B4 antigen and significantly decreased the proportion of My9+ and IL2-R-bearing cells. Southern analysis of DNA from leukemic cells revealed monoclonal rearranged heavy and k light-chain immunoglobulin genes. Immunoprecipitation of leukemic cell membrane proteins with a monoclonal antibody recognizing the IL2-R (anti-Tac) revealed a protein with the same molecular weight (55 kilodaltons) as that of the IL2-R present on PHA-stimulated normal T cells. Fresh leukemic cells did not express high-affinity IL2-R and did not proliferate in liquid culture or in a colony assay in the presence of recombinant IL2 (rIL2). PMA-treated blast cells, however, generated B cell colonies in the presence of rIL2, thus suggesting that PMA could induce functional IL2-R on immature leukemic B cells.