A brain-tumor model in adult monkeys may be significant because of the biological similarity to humans as well as the feasibility for surgical manipulation and for sequential computerized tomography (CT) scanning. In the present study, brain tumors were successfully produced in Japanese monkeys (Macaca fuscata), each weighing 2 to 10.8 kg, with an average age of 5.1 years old. Tumor cells were implanted by intracerebral inoculation of 4 X 10(7) chick embryo fibroblasts infected with the Schmidt-Ruppin strain of Rous sarcoma virus (RSV). With a 15- to 67-day latency, brain tumors were induced in 11 (73.3%) of 15 RSV-inoculated monkeys. Contrast-enhanced CT scans delineated all solitary intracerebral tumors greater than 4 to 6 mm in diameter. The CT images were proved at autopsy to be accurate within 2 mm in determining the size of tumor. Five of the 11 monkeys with intracerebral tumors died, with an average survival time of 26.6 days after RSV inoculation. The induced tumors were classified as either glioma or sarcoma by the presence or absence of glial fibrillary acidic protein (GFAP) and S-100 protein. A chromosome analysis of cultured tumor cells showed a diploid number of 42, indicating monkey origin. It is concluded that the reproducible brain tumor in the adult Japanese monkey inoculated with RSV can serve as a good experimental brain-tumor model for the further study of human malignant brain tumors.