AbstractThe 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 intra‐peritoneally 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 micro‐grams 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 rele