Antibody response to different doses of (T,G)-Pro--L, given in aqueous solution, was investigated in the high responder SJL and low responder DBA/1 strains by measuring hemolytic plaque-forming cells (PFC) in the spleens as well as hemagglutination titers in the sera. The gene responsible for the difference between the two strains in the response to this antigen, given in complete Freund's adjuvant, has been previously denoted Ir-3. This gene is not linked to the major histocompatibility locus. In the response to the optimal dose (1 mug) of antigen, no difference could be shown between the strains. The peak of the response and the numbers of direct and indirect PFC were similar in both strains in the primary and secondary response. After injection of higher doses (10-100 mug) of antigen, both the direct and indirect PFC responses were lower in the low responder than in the high responder strain. Moreover, the peak of the response occurred earlier in the high responder strain in the primary response to the 10 mu dose of antigen. After administration of a suboptimal dose (0.02 mug) of antigen, the low responder strain produced in the primary response 4-20 times more indirect plaques than the high responder strain. Also the number of direct plaques was higher in the low responder than in the high responder strain. The serum antibody responses to the optimal and higher doses of antigen were parallel to the PFC responses. From inhibition of PFC with free antigen, it was concluded that a similar proportion of cells was producing high and low affinity antibodies to (T,G)-Pro--L in both strains. High and low zone tolerance could be induced in the two strains with (T,G)-Pro--L, but no difference could be shown between the strains. It is suggested that the Ir-3 gene plays a role in the regulation of the balance stimulation and suppression according to the dose of antigen given.