The transfer of spleen cells from (C3H x C57Bl/6) F1 mice, capable of responding to (T,G)-A--L, into irradiated C3H parental recipients, normally incapable of responding to (T,G)-A--L, transfers the ability to make either a primary or secondary immune response to this synthetic polypeptide antigen. This localizes the genetic control of the ability to respond to the spleen cell population and indicates that the genetic control is exerted upon a process directly related to antibody formation. Studies with congenic strains of mice and linkage studies in segregating backcross populations show that the ability to respond to (T,G)-A--L and (H,G)-A--L is linked to the H-2 locus and can thus be localized to the IXth mouse linkage group.