SUMMARYThe Ag‐B allotype, mixed lymphocyte reactivity (MLR) and the immune response to poly(Glu52Lys33Tyr15) were assayed in male rats from the F2 hybrid and two back‐cross generations of the F344 and DA strains in order to investigate the structure of the rat major histocompatibility complex. No disparity between Ag‐B type and mixed lymphocyte reactivity was found in 263 animals. The immune response to poly(Glu52Lys33Tyr15) was closely linked to the Ag‐B locus, and both antibody production and the delayed hypersensitivity response were under polygenic control. These results suggest that the genetic loci which determine these responses in the rat are closely linked and that recombinational events between the Ag‐B and MLR loci are i