SUMMARYThere have been several reports of observations which suggest that transplantation tolerance may be a result of positive immunoregulation rather than simply unresponsiveness attributable to a lack of competent effector cells. In particular, several investigators have reported that tolerance of the H-Y and H-1 histocompatibility antigens is mediated by a population of thymus-derived lymphocytes. In a companion report, we have presented evidence that supports the existence of a suppressor cell to the H-Y antigen. Futhermore, we have observed that female mice rendered tolerant of the H-Y antigens by neonatal exposure to male lymphoid cells or by multiparity accept male skin grafts indefinitely, but inactivate male peritoneal exudate cells (PEC) in a second-set fashion. This observation has led us to investigate whether tolerance of other non-H-2 antigens is controlled by a similar mechanism. Using mice congenic with C57BL/10 at the H-4 and H-7 loci, we have shown that mice rendered tolerant of the H-7aand H-4bantigens by neonatal exposure to histoincompatible lymphoid cells are incapable of rejecting either skin or peritoneal cell allografts, suggesting that identical histocompatibility antigens are present on skin and peritoneal cells. Tolerance induced in neonatal mice to the H-4band H-7aantigens could not be adoptively transferred to syngeneic recipients. These results suggest that tolerance involving the H-4 and H-7 antigens is most likely because of a clonal inactivation of alloantigen-reactive cells as a consequence of neonatal exposure to antigen.