SUMMARY Rabbit anti-rat lymphocyte serum (ALS) was found to be capable of significantly delaying rejection of AgB incompatible rat heart grafts when given as a single injection as late as the 5th day after transplantation. Additional injection of ALS on the 2nd postgraft day resulted in only slight further delay of injection, but injection of normal rabbit serum on day 2 followed by ALS on day 5 yielded no immunosuppressive effect. This complete suppression of ALS activity is interpreted as evidence for production of antiglobulin antibody in response to normal rabbit serum followed by reaction of this antibody with ALS globulin injected later. Interference of attachment of antilymphocyte antibody to lymphocytes by pre-emptive reaction of host anti-IgG antibody with the heterologous antilymphocyte antiserum is proposed. Plasma cells producing anti-rabbit IgG were found in distant nodes 3 days after but not before normal rabbit serum injection.