Nonabsorbable radioactive material was used in treating malignant lesions by local injection. One hundred thirty-five Lewis Wistar rats were inoculated subcutaneously with tumor cells. After 7 days, the rats grew tumors of approximately 1 X 1 X 1 cm at the injection site. The rats then were divided into two groups; 50% were kept as a control group while the other 50% received single injections of 1 mCi yttrium 90 (90Y) microspheres directly into the center of the tumor. This study shows a significant reduction of the tumor growth rate in treated animals. Tumor sizes after 4 days averaged 1.31 ml in the treated rats and 9.74 ml in the control group. Excluding animals that had complete regression of the tumor, treated rats lived an average of 30.8 days from the day of treatment compared with the control rats, which lived an average of 17.4 days. Pathology examinations showed no effects from injected radioactive materials in the liver, bone marrow or the kidney. Examination of the injected area after 6 months showed that microspheres remained locally and that the tumor was replaced with collagen tissue.