SUMMARYThe primary and secondary immune response to the antigen bovine pancreatic α‐chymotrypsin was investigated in in inbred mice. It was found that strain differences in the immune response only became apparent after secondary immunization. The genetic control of the immune response was investigated in twelve different strains of mice, F1, F2and F1backcross hybrids, following secondary immunization. A continous distribution for the mean antibody responsiveness was obtained. High responsiveness was associated with both the H‐2 haplotype and three non‐H‐2 loci. Furthermore the F1hybrids produced a greater quantitative antibody response to chymotrypsin than either of the corresponding parental