The induction of skin tumors by chronic UV irradiation was compared in normal C3H/HeN(MTV⁻) mice, T-cell-depleted mice, and T-cell-depleted mice reconstituted with thymus grafts. The T-cell-depleted mice were thymectomized as young adults, lethally X-irradiated, and repopulated with neonatal liver cells. Skin tumors appeared earlier in the immunosuppressed T-cell-depleted mice than in the normal mice. However, tumors developed most rapidly in the group that was T-cell depleted and immunologically restored with thymus grafts. In the latter group, most of the tumors were squamous cell carcinomas, whereas in the normal mice, fibrosarcomas predominated. These experiments illustrated the limitations of this approach to studying the role of the immune response in carcinogenesis.