The lifespan of BDF mice with ascitic L1210 leukemia is more than doubled when they are treated with a single subcutaneously or intraperitoneally implanted sustained release cytosine arabinoside capsule. These capsules are drug-polymer composites, with dimensions of 1.0 cm in diameter and 0.15 cm in thickness, and a drug content of 17 +/- 3 mg each. Blank silicone rubber discs implanted intraperitoneally or subcutaneously in the leukemic mice produced no beneficial effects. Control healthy mice receiving the ARA-C releasing capsules continued to live with no obvious signs of drug toxicity. In vitro release of ARA-C from the discs was measured. Drug release was maintained above 20 micrograms per day for more than 35 days. ARA-C extracted from discs recovered from implant studies showed that less than 16% of the total drug content was released in vivo.