Abstract Dendritic cells are very potent, if not the most effective, stimulator cells for the induction of primary T cell immune responses. We have established, from murine fetal skin, growth factor-dependent cell lines with a pronounced dendritic shape and a phenotype similar to that of fetal Langerhans cells (i.e., MHC class I+/II). Functionally, these lines induce a vigorous proliferation of allogeneic, but not syngeneic, CD8+ lymphocytes. T cell blasts thus generated are capable of lysing various target cells in an MHC class I-restricted fashion. Our contention that this skin cell-induced MHC class I-restricted activation of CD8+ lymphocytes occurs in the absence of CD4+ T cells is based on 1) the lack of FACS-detectable CD4+ T cells in the purified CD8+ T cell population, 2) the lack of reactivity of purified CD4+ T cells to MHC class I-disparate fetal skin cell lines, and 3) the inhibition of the fetal skin cell-induced MLR by anti-CD8/MHC class I, but not anti-CD4/MHC class II, mAb. Skin cell-induced activation of unprimed CD8+ T cells was found to be critically dependent on physical contact between stimulator and responder cells and the expression of the costimulatory molecule B7 on fetal skin cell lines. Lines described in this study may represent a powerful tool for studying the molecular events occurring in the induction of MHC class I-restricted primary immune responses, understanding their pathophysiologic role, and perhaps may prove useful for vaccination purposes against selected pathogens.