Acute leukemias are believed to arise from unregulated proliferation of hematopoietic cells and loss of the ability to differentiate. Studies on the immunophenotypes of leukemic cells are very helpful for the understanding of antigen expression during normal hematopoiesis. CD7 antigen has until recently been considered to be a T-cell marker but has been found to be expressed by leukemic cells from some acute myeloid leukemia (CD7+ AML) and the normal putative counterparts of blasts from CD7+ AML can be found in human fetal livers. These double CD7 and myeloid antigen (CD13 and/or CD33) positive progenitors tend to lose their CD7 expression, while retaining their myeloid characteristics, after in vitro culture with the differentiation-inducing agent phorbol ester (TPA). This suggests that the cells are probably committed to the myeloid cell lineage and that CD7 is only transiently expressed in the early differentiation stage. On the other hand, there is a subset of CD7+ hematopoietic precursors which lack mature myeloid and T-cell antigens and have the potential to differentiate to both T-lymphoid and myeloid cells. These cells may in fact be the common myeloid-T lymphoid progenitors and represent the normal counterparts of acute undifferentiated leukemia or minimally differentiated T-cell acute lymphoblastic leukemia.