In a case of acute promyelocytic leukemia (APL), the expression of terminal deoxynucleotidyl transferase (TdT), an early lymphoid marker, was detected. Double-fluorescent staining for the myeloid-specific antigens VIM-2 and VIM-D5 in combination with specific antiserum for TdT suggested a mixed leukemic cell population consisting of a morphologically, cytochemically, and immunologically promyelocytic component (80%) and a lymphoid, TdT+ component (20%) that was myelomonocytic in morphology but otherwise without any evidence of nonlymphoid nature. Fluorescent-activated cell analysis revealed that a greater number of cells reacted with monoclonal anti-T antibodies (OKT3, OKT6, and OKT11) than could be identified as lymphoid by TdT expression. As confirmed by double-staining fluorescence microscopy, a large fraction of the promyelocytic leukemia cells were biphenotypic, expressing both myeloid and lymphoid markers (50% positive for VIM-D5 and OKT6, 30% positive for VIM-D5 and OKT3). Subsequently, in vitro differentiation experiments were performed. While treatment of the cells with GCT-conditioned medium favored proliferation, with only a weak and delayed promotion of the cells towards maturation as reflected by enhanced expression of the mature T-marker T3 but persistent expression of the thymocyte antigen, exposure to all-trans and 13-cis retinoic acid resulted in marked differentiation of both the myeloid and the lymphoid cell characteristics. Retinoid treatment resulted in the loss of TdT, a partial disappearance of the T6-antigen, and the expression of the late T cell antigen T3 by almost 70% of the cells. In addition, myeloid maturation was obvious from the morphologic appearance of the cells, as well as from the expression of the OKM1- associated antigen by a majority of the cells. This report concerns a unique case of APL in which, for the first time, a coexistence of promyelocytic and lymphoid elements was detected, with exposure of the cultured leukemic cells to retinoic acid inducing maturation along both the myeloid and the lymphoid lineage.