The primary in vitro antibody response to the type 2 antigen, trinitrophenyl (TNP)-Ficoll, is controlled by two complementing loci in the H-2 region of the mouse major histocompatibility complex (MHC). High responder alleles at both loci are necessary for a high responder phenotype. Previous studies mapped one locus of control to the I-A subregion. In this report we demonstrate by recombinant analysis that the second locus of control is located between the H-2S and D regions. A comparison of responses in the B10.BAR6, B10.BAR10, and B10.BAR11 strains defined a locus controlling the response to TNP-Ficoll in a single haplotype, bounded on the left by the crossover event in the B10.BAR10 and on the right by the crossover event in the B10.BAR6 strain. A monoclonal antibody directed against this right-hand region of control has been produced (48.21.7) that blocks the response to TNP-Ficoll at the level of the antigen-presenting cell. The monoclonal antibody 48-21.7 is specific for the high responder b allele at the right-hand locus and did not inhibit responses to other protein antigens tested. The immune response to TNP-Ficoll was not inhibited by monoclonal antibodies that react with H-2Db or Qa-2 specificities, suggesting that the TNP-Ficoll response is controlled by a unique locus located between H-2S and D. Finally, 48-21.7 recognizes and precipitates a unique product of approximately 40,000 mol wt that is distinct from the H-2D region product recognized by the monoclonal antibody B22/249.