The Contractor shall prepare anti-receptor antibody (ARA), measure the effect of reagent ARA on established immune responses, measure the effect of antibody and reagent ARA on in vitro tests of cytotoxicity, and test the effect of ARA on grafted tumors. Experiments shall be carried out to investigate whether ARA can alter the course of antibody response, graft versus host reaction, or graft rejection. The hypothesis that ARA suppresses immune responsiveness by combining with receptor to block stimulation by antigen of reactive cell in adults or by depleting or eliminating clones of reactive cells in neonates shall be investigated.