We have examined the expression and biological effects of class-I MHC molecules on the immune response to the line I lung carcinoma. The line I system is of interest because these tumor cells have very low constitutive levels of class-I molecules but can be induced to express levels found on spleen cells, by culturing the cells with agents such as dimethylsulfoxide (DMSO) or interferon gamma (IFN-gamma). This induction is significant immunologically, since induced cells can be lysed very effectively by cytotoxic T lymphocytes (CTL), whereas the uninduced cells cannot. CTL clones that are reactive with line I cells have been generated and used in vitro and in vivo, to examine the interactions of T cells with line I. We have shown that the expression of class I on tumor cells is induced in vivo by IFN-gamma, and that this induction is associated with the ability to reject the tumor. We will also introduce preliminary data concerning the mechanism of induction in which CTL appear to induce class-I MHC both in vitro and in vivo. The results are discussed in terms of a model which may be important generally for class-I inducible tumors.