The primary in vitro antibody response to TNP-Ficoll was found to be under H-2-restricted Ir gene control. Strains B10(H-2b), B10.A(H-2a), and B10.S(9R) (H-2t4) were consistently low responders while strains D2.GD(H-2g2), B10.GD(H-2g2), and B10.S(H-2s) were high responders. The in vitro TNP-Ficoll response in congenic recombinant and F1 hybrid mice demonstrated the requirement for complementation of two independent Ir genes. One Ir gene mapped in or to the left of the I-A subregion with high responder alleles being s or d. The second Ir gene mapped to the right of the I-E subregion and required b or s alleles for complementation. These results were further supported by the ability to block the TNP-Ficoll response by appropriate anti-Ia serum pretreatment of the antigen-presenting macrophages. When a structurally different polysaccharide antigen TNP-dextran was used, an identical pattern of restriction was observed.