IL-17 is a potent proinflammatory cytokine produced by activated memory T cells. Recent studies in both human autoimmune diseases and in their animal models have indicated that IL-17 rather than IFN-γ might be the essential T-cell effector cytokine in the T-cell mediated autoimmune process. Since the thymus has a central role in maintaining T-cell self-tolerance and disturbance of thymic self-tolerance is implied in various autoimmune diseases, we here investigated the capability of murine thymocytes to produce IL-17. Our results indicate that thymocytes are a potent source of IL-17 in response to CD3 stimulation and various microbial immune stimuli and thereby show different patterns in the expression of the proinflammatory cytokines IFN-γ and IL-17. In addition, strong differences between thymocytes and splenocytes were detected. Altered IL-17 production by thymocytes upon contact with foreign pathogens might be a key regulator in the education of adaptive immunity.