We have been studying gene expression in cytotoxic immune cells with specific emphasis on large granular lymphocytes (LGLs) and cytotoxic T cells. We have been characterizing a new gene which appears to be a receptor for LGLs and is expressed in the cytoplasm of T cells. We have also correlated cytotoxic function with expression of the cytotoxic protein, perforin, and have identified cytokines which upregulate perforin mRNA expression. These studies begin to define, at the molecular level, the genes involved in the development and maturation of cytotoxic immune effector cells.