Five-day-old Fischer 344 rats were inoculated intracerebrally with BCG cell walls attached to oil (BCG-CW) or with oil emulsion alone. Doses of BCG-CW of 40 μg in 0.01 ml, 200 μg in 0.05 ml, and 400 μg in 0.1 ml were used, with similar volumes of oil emulsion for comparison. Twentyfive days later hydrocephalus, intracerebral granulomas, and arachnoid granulomas were found at autopsy. The frequency and severity of these abnormalities were related to the presence of BCG-CW in the inoculum and the dose administered. There were minimal abnormalities with 40 μg BCG-CW. Two microliters of Avian sarcoma virus (ASV) suspension was inoculated intracerebrally into 5-day-old rats to induce intracerebral gliomas. One hour later 40 μg BCG-CW in 0.01 ml, 80 μg BCG-CW in 0.02 ml, or similar volumes of oil emulsion alone were inoculated intracerebrally in the region of the virus injection site. BCG-CW or oil emulsion alone failed to prevent induction and growth of the ASV-induced brain tumors. Survival curves of the BCG-CW inoculated animals were similar to that obtained for control animals injected with ASV and otherwise untreated. The oil emulsion decreased survival times compared with control animals.