Spleens of adult mice of the A strain background that were rendered tolerant as neonates of class II alloantigens only (A.TH tolerant of A.TL, A.TL tolerant of A.TH) contain large numbers of tolerogen-responsive T cells, many of which secrete IL-4, but not IL-2. Since these spleens also contain suppressor cells that can adoptively transfer skin allograft acceptance in vivo and can prevent generation of class II-specific cytotoxic T cells in vitro, it is important to determine the origins during postnatal development of these cells. Class II disparate, semiallogeneic haematopoietic cells were injected into newborn A.TH and A.TL mice. Periodically thereafter (1 to 60 days post-injection, but prior to challenge with a tolerogen-bearing skin graft), thymocytes and splenocytes from these mice were examined in vitro for tolerogen-specific reactivity in mixed lymphocyte reactions during which proliferation and IL-2 and IL-4 production were assayed. Within 24 hours of neonatal injection, the thymus and spleens of injected mice were profoundly depleted of tolerogen-responsive T cells. However, there was no commensurate loss of I-E-related V beta 5+ cells in the thymus of A.TH mice that received neonatal inoculations of I-E-bearing A.TL cells. During the ensuing weeks, tolerogen-responsive proliferative and IL-4-secreting T cells were detected in thymus and spleen. However, not until after the mice were more than 60 days of age were tolerogen-responsive cells able to secrete IL-2. Since physical clonal deletion of tolerogen-related V beta 5+ cells is a characteristic of neither neonatal nor adult A.TH and A.TL mice that received injections of semiallogeneic cells at birth, and since tolerogen-responsive IL-4 producing cells exist in adult mice that have permanently accepted (A.TH x A.TL)F1 skin grafts, our results imply that the tolerogen-responsive T cells detected in adult tolerant mice are descendants of the novel IL-4-producing T cells that arise in the thymus almost immediately after the tolerance conferring inoculum of semiallogeneic cells. The possible mechanisms responsible for generation of IL-4-producing, tolerogen-responsive T cells and the role of these cells in maintenance of tolerance of class II alloantigens are discussed.