Using a radioimmunoassay, increased levels of a human thymus/leukemia-associated antigen (HThy-L) have been detected in leukemic cells and plasma from most patients with E-rosette-positive acute lymphoblastic leukemia (ALL) and a number of patients with E-rosette-negative ALL, acute myeloblastic leukemia (AML), acute monomyelocytic leukemia (AMML), and acute undifferentiated leukemia (AVL). Low levels of HThy-L have been demonstrated in white cells from patients with chronic myelocytic leukemia (stable phase) and in mononuclear cells from patients with chronic lymphatic leukemia. The relationship between HThy-L and differentiation of hematopoietic cells is discussed.