[B10.A(4R) × B10.PL]F₁ mice are low responders to pigeon cytochrome c, while [B10.A(2R) × B10.PL]F₁ and B10.A mice are high responders. The in vivo site at which the different aliomorphs of the E_a Ia molecule exert their Ir gene effect on the immune response to pigeon cytochrome c was examined by creating two different sets of radiation-induced bone marrow chimeras. [B10.A(4R) × B10.PL]F₁(b.m.) → B10.A(lrr.) chimeras, which possess antigen-presenting cells (APC) of the low responder, but whose T cells are educated in a high responder environment, were found to be low responders to pigeon cytochromec. In contrast, B10.A(b.m.) → [B10.A(4R) × B10.PL]F₁(lrr.) chimeras, which possess APC of the high responder type, but whose T cells are educated in a low responder environment, responded to pigeon cytochrome c Addition of B10.A APC to the first type of chimera, both prior to antigen priming and at the time of the secondary challenge in vitro, converted 50% of the animals to responders Furthermore, [B10.A(4R) × B10.PL]F₁ mice responded to pigeon cytochrome c If they were primed with a 10-fold greater antigen dose and restimulated in vitro In the presence of B10.A APC. These results suggest that the primary site of the Ir gene defect in this system is at the level of antigen presentation and not in the T cell repertoire.