A case of acute lymphoblastic leukemia with a giant splenomegaly in an 8-year-old boy was investigated by immunological, cytochemical and electronmicroscopical techniques. Bone marrow and peripheral blood were largely replaced by large blast cells with a nonconvoluted nucleus. Cytochemically, most of the blast cells showed strong focal acid phosphatase activity. In the surface marker studies, these blast cells formed EAC rosettes but not E rosettes, while they showed positive reactivity with anti-T lymphocyte serum but not with anti-B lymphocyte serum. This membrane phenotype E-, C3R+, T+, B- suggested that leukemic blast cells in this patient presumably originated from precursor T cells.