A 5-yr-old girl with acute lymphocytic leukemia presented with moderate hepatomegaly, marked splenomegaly, but no evidence of a mediastinal mass. The peripheral blood white count was 270 x 10(9)/liter with 99% leukemic cells. Surface marker analysis showed the lymphoblasts to be E- rosette negative and complement receptor positive. The patient's leukemic cells were unreactive with anti-p23,30, which detects Ia-like antigens, and strongly reactive with A99 anti-T-cell serum, which reacts with normal human thymocytes and peripheral blood T cells. The percentage of leukemic cells bearing complement receptors diminished during relapse. The leukemic cells obtained at diagnosis and during relapse were nonreactive to mitogens and alloantigens and failed to stimulate proliferation of normal lymphocytes in mixed lymphocyte culture. There was no evidence for active suppression of normal lymphocyte reactivity mediated by the leukemic cells. The surface marker and functional profile of these leukemic cells is consistent with that of an early stage in T-cell maturation.