T-cell-deficient mice, either anti-thymocyte serum treated or nude mice, were immunized with hapten (4-hydroxy-3,5-dinitrophenyl acetic acid, NNP) conjugates of syngeneic, allogeneic, or xenogeneic erythrocytes. Immunization with syngeneic conjugates led to a stronger anti-NNP response than immunization with allogeneic or xenogeneic conjugates. A study of congenic mouse strains suggested that a...