A series of 21 patients (5 refractory anemias with an excess of blasts in transformation and 16 acute leukemias) were treated with small doses of ARA-C (10 mg/sq m/12 hr for 15-21 days). Improvement was noted in 15 cases (71%) and complete remission observed in 12 (57%). Complete remission was obtained after one course of treatment in 8 cases. The fact that these patients entered remission relatively slowly and did not suffer marrow aplasia suggests that low-dose ARA-C may function in vivo as it does in vitro, i.e., by inducing differentiation of leukemic blasts.