Rabbit anti-hamster brain sera prepared from the brains of Syrian and Chinese hamsters were evaluated for their potential to identify thymus-derived lymphocytes present in these animals. The unabsorbed antisera were cytotoxic in vitro for both homologous and heterologous thymocytes and splenocytes. Following absorption with hamster liver and erythrocytes, the antisera remained toxic for thymocytes, but were less toxic for splenocytes. Comparative testing of both the rabbit anti-Syrian and anti-Chinese brain sera with rabbit anti-C3H mouse brain serum indicated that considerable cross-reactivity exists between the different anti-brain sera. The in vivo administration of any of the antisera to C3H mice resulted in a lowered titer to the thymus-dependent sheep erythrocyte antigen response as measured by the hemolysis-in-gel technique. In vitro treatment with the anti-brain sera had no effect on plaque-forming cells present in the spleen.