Activated T cells in spontaneous lupus presumably bypass normal tolerance mechanisms in the periphery, since thymic tolerance appears intact. To determine whether such T cells indeed avoid in vivo peripheral tolerance mechanisms, we assessed their activation and recall responses after in vivo Ag stimulation in the absence of exogenously supplied costimulatory signals. Naive CD4+ AND (transgenic mice bearing rearranged TCR specific for pigeon cytochrome c, peptides 88–104) TCR-transgenic T cells, specific for pigeon cytochrome c, from lupus-prone Fas-intact MRL/Mp+ Fas-lpr and from H-2 k-matched control CBA/CaJ and B10. BR mice (MRL. AND, CBA. AND, and B10. AND, respectively) were adoptively transferred into (MRL× CBA) F 1 or (MRL× B10) F 1 recipients transgenically expressing membrane-bound pigeon cytochrome c as a self-Ag. MRL. AND and control CBA. AND and B10. AND-transgenic T cells were activated and divided after transfer, indicating encounter with their cognate Ag; however, T cells from CBA. AND and B10. AND mice were impaired in their ability to proliferate and produce IL-2 after challenge with pigeon cytochrome c in ex vivo recall assays, a typical phenotype of anergized cells. By contrast, MRL. AND T cells proliferated more, and a significantly higher percentage of such cells produced IL-2, compared with control T cells. This observation that MRL T cells avoided anergy induction in vivo was confirmed in an in vitro system where the cells were stimulated with an anti-CD3 in the absence of a costimulatory signal. These experiments provide direct evidence that CD4+ T cells from Fas-intact lupus-prone MRL mice are more resistant than nonautoimmune control cells to anergy induction. Anergy avoidance in the periphery might contribute to the characteristic finding in lupus of inappropriate T cell activation in response to ubiquitous self-Ags.