These studies were designed to determine what influence prior immunization with homologous H isoantigens might have on the subsequent proliferative activity of lymphocytes from these animals in the mixed lymphocyte interaction. The results demonstrate the following: (a) Subcutaneous immunization with splenic cells from donors differing at the major H locus accelerates the tempo of the proliferative response to F(1) cells bearing these same antigens in the MLI, whereas antigen given systemically reduces the proliferative response. (b) The altered proliferative behavior is specific for the immunizing antigens. (c) The period after immunization during which the MLI displays an altered tempo is a short one, lasting not longer than 3 wk. (d) Whether they are derived from previously immunized or from normal donors, the proportion of lymphocytes responsive in the MLI is the same, even though the response profiles are different. These results suggest that in comparison to immune responses to other types of antigens, immunologic reactivity to the major H isoantigens already involves a large number of antigen-reactive cells in normal animals and that the proportion of these cells is not increased as a result of immunization. Rather, lymphocytes from immunized animals respond more rapidly to the presence of these antigens.