Tumor-specific transplantation immunity was demonstrated in syngenic rats immunized and challenged with tumor cells (T9) from an N-methyl N-nitrosourea-induced brain tumor. All unimmunized rats and rats pretreated with normal glial cells in complete Freund's adjuvant or with adjuvant alone died of intracerebral (i.c.) neoplasia at approximately 3 weeks post-challenge. Rats which were immunized with T9 cells with or without adjuvant failed to develop i.c. tumors by 6 weeks even though the challenge dose was 2 logs greater than a dose which consistently resulted in neoplasia in unimmunized rats.