Among the differences observed between the various high (H) and low (L) antibody responder lines of mice resulting from distinct bidirectional selective breedings, one of the most puzzling is the variation in the "multispecific effect," i. e., in the modification of antibody responses to antigens unrelated to those used during the selection. The best examples are the H and L lines of selection IV, selected on the basis of responses to somatic antigen of Salmonella which do not differ in their antibody responses to sheep erythrocytes (SE). However, a wide range of variability is observed in the responses of (H w ร LIv)F2 hybrids to this antigen, and it was therefore hypothesized that distinct groups of genes might regulate antibody responses to SE and the somatic antigen. Indeed, a new selection (IV-A) for anti-SE responsiveness started from these (H~v x LIv)F 2 successfully produced a high and a low anti-SE responder line. The results of selection IV-A and the variance analysis of (HIv_ a ร LIV_A)F 2 hybrids are reported. They are roughly similar to those in selection I, also carried out for anti-SE responsiveness. In vivo attempts to identify the major regulatory mechanism which contributes to the interline difference indicate that the efficiency of macrophage accessory function has been modified in selection IV-A, as was observed in selection I, whereas this function did not differ in H~v and L w lines. Probably in relation to the involvement of macrophage function there is a notable increase of the multispecific effect in selection IV-A when compared with selection IV. The results of selection IV-A demonstrate that responsiveness to heterologous erythrocytes and to somatic antigen of Salmonella are under Abbreviations used in this paper: f. ag. S., flagellar antigen of Salmonella; H, high responder line (roman numeral is the selection number); h2, heritability; HE, human erythrocytes; HoGG, horse gamma globulin; L, low responder line (roman numeral is the selection number); PE, pigeon erythrocytes; R, response to selection; RGG, rabbit gamma globulin; S, selection differential; s. ag. S., somatic antigen of Salmonella; SE, sheep erythrocytes Address correspondence and offprint requests to: D. Mouton separate polygenic control operating through distinct regulatory mechanisms. The choice of the selection antigen and immunization procedure is of major importance for defining the gene interaction operating in each selective breeding experiment and the extent of its multispecific effect.