The potential usefulness of cytosine arabinoside (ara-C) in the treatment of T-cell acute lymphoblastic leukemia (T-ALL) was studied by measuring the influx of ara-C into the lymphoblasts and the conversion of ara-C to its triphosphate form (ara-CTP) in lymphoblasts from the bone marrow of eighteen patients of T-ALL. Intracellular accumulation of ara-CTP was 3.5 times greater in T-cell lymphoblasts than in non-T or AML blasts. The increased formation of ara-CTP in T-ALL cell may be correlated with the good clinical response of T-ALL patients to the treatment with ara-C in combination with mitoxantrone.