The rejection of murine UV-induced skin cancers by normal mice is a striking example of powerful immune surveillance of the normal host against malignant cells. In this study, we show that UV-induced regressor tumors regularly grew progressively and killed mice that were depleted of CD8⁺ T-cells. Depletion of CD4⁺ T-cells had no effect, suggesting that CD8⁺ but not CD4⁺ T-cells were required for this immune surveillance. To determine whether change in major histocompatibility complex (MHC) class I expression was a frequent event that caused low immunogenicity of tumors or facilitated escape from immune destruction, recently isolated murine tumors of varying degrees of immunogenicity, including highly immunogenic UV-induced regressor, less immunogenic UV-induced progressor, and poorly immunogenic spontaneous progressor tumors, were compared. There was no correlation between the ability of a tumor to grow progressively in a normal immunocompetent host and the level of constitutive class I expression or the level of expression induced in vitro by γ interferon. (Only 1 of more than 20 progressor tumors analyzed showed complete loss of a MHC class I molecule.) Some progressor variants showed loss of a unique tumor-specific cytotoxic T-lymphocyte-defined antigen, consistent with earlier evidence of antigen loss providing a mechanism for tumor escape. However, most of the host-selected progressor variants retained both MHC class I antigens and the unique tumor antigens that we could detect with cytotoxic T-lymphocyte clones, suggesting that mechanisms other than loss of MHC class I or of the unique target antigen may be involved in escape of some tumors from a highly effective CD8-dependent host surveillance.