On the basis of in-vitro studies indicating that low concentrations of cytosine arabinoside exert preferential inhibition of granulocyte-macrophage colony-forming cells from patients with chronic granulocytic leukemia vs normal subjects, we treated two outpatients with low doses of this agent, administered by subcutaneous infusion for 12-31 days. Both patients continued their usual activities, including employment, during these infusions. They exhibited only Ph-positive metaphases at entry into the protocol but in both cases, Ph-positive cells were reduced to approx. 10% of marrow metaphases, after 2-3 successive infusions. Both patients exhibited significant increases in Ph-positive cells, to 46 and 72% of marrow metaphases, during subsequent chemotherapy with hydroxyurea, in dosage sufficient to maintain granulocytopenia and a normal serum B12 level. After additional cytosine arabinoside, both patients again showed decreases in Ph-positive cells, to 7% (p less than 0.01) and 19% (p less than 0.0001), respectively. This clinical experience is consistent with the conclusion that cytosine arabinoside (but not, hydroxyurea) exerts a selective antileukemic effect in some patients with CGL.