Neonatally induced tolerance of class II MHC alloantigens in mice of the A/J strain background is achieved without evident clonal deletion, because lymphoid cells from tolerant animals proliferate in mixed lymphocyte reactions against tolerogen-bearing stimulators. However, the pattern of lymphokines produced in bulk cultures by these tolerogen-reactive cells is unusual in that, unlike lymphocytes from nontolerant normal mice, lymphocytes from tolerant mice produce measurable amounts of IL-4 when stimulated with tolerogen. Using limit dilution analysis, we have determined that the ability to detect IL-4 in bulk cultures is caused by a high frequency of precursors of Ag-specific IL-4 secreting (pIL-4) cells among tolerant, but not normal, responder lymphocytes. Moreover, after repeated in vitro exposure to class II alloantigens, and after in vivo grafting with class II-disparate skin, the frequency of pIL-4 cells rises among lymphocytes from normal mice, eventually reaching levels similar to those found in class II tolerant mice. Because in normal mice a high frequency of class II-reactive pIL-4 cells reflects the "primed" state, we conclude that a subset of T cells that resemble Th2 cells is similarly primed in neonatally tolerized mice. We propose that this priming is achieved by the neonatal inoculation of semiallogeneic bone marrow cells. Because no expansion in the clone size of tolerogen-specific precursors of Ag-specific IL-2-secreting T cells (pIL-2) is observed among tolerant animals, and because the pIL-2 cells from tolerant mice are incapable of differentiating into cytotoxic and delayed hypersensitivity effector cells, we further propose that the precociously primed pIL-4 cells function as "suppressor" cells, helping to maintain the tolerant state.