Neonatal transplantation tolerance was one of the first experimental systems to reveal that tolerance could be achieved to non-self antigens in living animals. Functional and direct evidence (obtained by the use of monoclonal antibodies directed at Tcell receptors specifically reactive with I-E molecules) confirm that tolerance is achieved, at least in part, via clonal elimination of developing thymocytes. In this report, we show that induction of tolerance of class I alloantigens in neonatal mice is governed by expression of I-E molecules. Neonatal I-E non-expressor mice proved to be highly resistant to the acquisition of class I tolerance if the donor inoculum expressed disparate class I antigens as well as I-E molecules. The spleens of the few class I-tolerant, I-E non-expressor mice that were generated were found to be depleted of I-E-reactive (RR315+) T cells, whereas no such depletion was observed in their neonatally injected, but non-tolerant littermates. By contrast we found no resistance to tolerance of I-A alloantigens when neonatal I-E non-expressor mice received injections of I-A-disparate, I-E-bearing donor cells. In these tolerant mice, splenic I-E-reactive T cells were readily detected in apparently normal amounts. These results indicate that lack of I-E expression in newborn mice confers resistance to tolerance induction to class I alloantigens, especially when the latter are expressed on donor cells that also display I-E molecules. The possible mechanisms operating to produce resistance to tolerance induction in neonatal mice are discussed, including the possibilities that (a) I-E may act as a restricting element during tolerance induction (an ontogenic process), and (b) the expression of I-E on H-2-disparate, I-E-expressing test skin allografts may provide a source of "help" for CD8+ cytotoxic T cell precursors, leading to graft rejection.