IT has been shown that massive doses of allogeneic serum produce the prolongation of renal allograft survival in adult pigs1. Furthermore, alloantigenic activity was demonstrated in human serum by inhibiting certain reactions in vitro2-6; and in some strains of mice, the presence of histocompatibility (H) antigens in the serum could be demonstrated by inhibition of the cytotoxicity of specific alloantisera7.