An effective method was sought to immunize rats against the growth of intracerebrally (IC) injected T9 tumor, a gliosarcoma cell line. Rats which were immunized with either 10(6) T9 cells mixed with 0.14 mg C. parvum, or 10(7) irradiated T9 cells showed tumor immunity to intradermal (ID) transplantation. However, to obtain tumor immunity to an IC challenge of T9 cells, rats initially had to reject an ID challenge of T9 cells. After sequential rejections of ID challenges of as many as 10(7) cells, a high degree of immunity was obtained, allowing rejection of up to 5 X 10(6) T9 cells injected IC. Spleen cells from highly immunized rats mixed with T9 cells at a ratio of 1:25 (tumor:spleen) destroyed T9 tumor cells in a Winn test. Normal spleen cells had no effect on tumor growth. We conclude that: 1) T9 cells are moderately immunogenic, 2) an effective method of immunization of CDF rats against ID transplanted T9 cells is 10(6) T9 cells with 0.14 mg C. parvum or 10(7) irradiated T9 cells, 3) a higher degree of tumor immunity is necessary to reach the brain than is needed to reach the periphery, and 4) spleen cells of highly immunized rats are cytotoxic to T9 tumor cells.