In addition to allowing for the long-term culture of both murine and human cytolytic T lymphocytes, T-cell growth factor (TCGF) functions as the key proliferation-inducing second signal in both T-cell antigen sensitization and mitogenesis. The observation that thymocytes responded normally to T-cell mitogens in the presence of TCGF, prompted the investigation of the effect of TCGF on nude mouse lymphocyte responses in vitro. We found that spleen, lymph node, and bone marrow cells, isolated from nude mice, were incapable of producing TCGF yet responded normally to T-cell mitogen sensitization provided stimulation was conducted in the presence of TCGF. Nude mouse spleen cells were also capable of responding to alloantigen sensitization in mixed lymphocyte cultures (NLMC) conducted in the presence of TCGF. Thy-1 antigen-positive cells harvested from TCGF-supplemented nude mouse MLC effectively mediated the cytolysis of alloantigen-specific target cells as tested in standard 51Cr-release assays. Cytolytic nude mouse effector cells have remained in TCGF-dependent culture for over 3 mo during which they have continued to mediate significant levels of alloantigen-specific cytolytic reactivity. These results suggest that prothymocytes present in nude mice are capable of responding to immunologic stimuli by differentiating, in vitro, into cytolytic T lymphocytes and that furthermore, a major function of the thymus may be to effect the maturation of TCGF-producing cells.