Amount and affinity of anti-beta-D-galactosidase-activating antibodies (AAb) were studied in two lines of mice obtained by selective breeding for antibody response to heterologous erythrocytes. These lines, referred to as high (H) and low (L) lines, differ also in their responses to many antigens. Concerning AAb amounts, the maximal titer in H mice is higher than in L mice. However, the AAb levels in H mice decreased with increasing immunizing doses and became lower than in L mice for the highest dose. This fact can be explained in terms of intramolecular competition or suppression. Whereas the usual maturation pattern was observed in H mice, no AAb of high affinity were found in L mice, even after a minimal immunizing dose. At any dose tested, no maturation of AAb occurred until three months after immunization. This deficiency may be ascribed to B lymphocyte or macrophage modifications. A correlation between the deficiency of high-affinity AAb in L mice and their genetically induced low antibody responsiveness cannot be proved.