Mouse sera contain "natural" antibodies directed exclusively against polymorphic--non-histocompatibility--antigens of the chicken Ea- A blood group system.

Natural agglutinating antibodies in normal sera from A and B10 mice react exclusively with antigenic determinants of the polymorphic Ea-A blood group system (a non-histocompatibility system) of the chicken. Agglutinating activity of these "natural" antibodies, which was demonstrated in the IgM fraction of the serum, did not correlate with the cytotoxic activity against chicken cells in A mice compared to B10 mice. "Natural" agglutinating antibodies against antigens A13 and A14 present in chickens of congenic lines were demonstrated in NMS from A mice. On the other hand, B10 mice showed no antibody reactivity against A13 antigen. The antibody reactions against antigen A13 could be induced in B10 mice by immunization with the respective erythrocytes. In this case, the antibody activity was concentrated in the IgG fraction of the serum. After immunization of B10 mice, agglutinating antibodies against antigenic determinants of the Ea-B (MHC) system appeared, which were not detected in NMS.