Liver absorbed rabbit anticanine brain serum (ABS) was tested for its immunosuppressive capacities in dogs. Single intravenous injection of ABS led to significant decrease in lymphocyte counts and functions after 24 hours, as measured by phytohemagglutinin stimulation and mixed lymphocyte culture. The immunosuppressive effectiveness of ABS could further be documented by prolongation of skin graft survival in unrelated dogs which were treated daily for 12 days with intramuscular ABS injections. The ABS was generally well-tolerated. One dog died of pneumonia. All dogs showed moderate decrease in platelet counts after 10 days of ABS treatment. Autopsies showed normal cerebral histology, moderate lymphoid hypoplasia, and no changes in the bone marrow and liver. The results suggest that liver absorbed anti-canine brain serum is a potent immunosuppressant with limited toxicity.