SUMMARYThe immune response to the antigen bovine pancreatic α‐chymotrypsin was investigated in ten recombinant strains of mice. Using a fixed antigen‐percentage bound isotope technique, it was found that the quantity of antibody produced was related to the H‐2 haplotype of the responding animal. A continuous distribution for the mean antibody responses was obtained for the ten strains of mice. High responsiveness was associated with the H‐2 haplotypey2. The genetic control of the immune response to this immunogen was found to be both quantitative and qua