Several distinct chromosomal segments were recently identified by cosegregation analysis of polymorphic markers with antibody responsiveness in an F2 cross between high (H) and low (L) antibody responder lines of Biozzi mice. The effect associated with the relevant markers has now been investigated in backcross populations (toward the L line) bred from H and L mice made coisogenic at the H-2 locus. The antibody titers, measured on days 5 and 14 of the primary response to sheep red blood cells, were considered to be two distinct quantitative phenotypes. The results of single or multilocus analyses demonstrated the significant involvement, at one or the two titration times, of Im gene(s) on four distinct chromosomes: 4, 8, 12, and 18. The regions on chromosomes 6 and 10 have a lesser but still suggestive effect. The contribution of each locus ranged from 3% to 13%, and together these loci accounted for about 40% of the phenotypic variance at each titration time. The data are compatible with an additive effect of the relevant loci and suggestive of some interaction effects. In a second backcross toward L line, the H line alleles of the putative Im genes on chromosomes 6, 8, and 12 were isolated from each other and their effects were still detected.