The objectives of this project are unchanged from the original proposal and include in the second year: (1) the production of monoclonal antibodies that inhibit the in vitro cytolytic activity of human cytotoxic T-cell lines; and (2) characterization of lymphocyte function associated antigens (LFA) in the human cytotoxic T-cell reaction. Mice and rats have been immunized with longterm human cytotoxic cells and antigen preparations derived from these cell lines, and splenocytes from such animals will be fused with X63 myeloma cell line. Primary screening system for identifying monoclonal antibodies will be by inhibition of the 51Cr-release microcytotoxicity assay and will include positive controls for blocking. LFA-1 and T8 surface molecules will be extensively characterized for their function in the cytolytic reaction, using monoclonal antibodies and cloned human CTL lines. In addition, we have studied cloned CTL lines of both the T4 and T8 subsets which exhibit either antigen-specific or natural killer-like activity and have shown that these cells are capable of releasing in vitro a cytotoxic lymphokine(s). These lymphokines exhibit selectivity in their ability to lyse tumor cells but not normal cells. Biochemical characterization of this cytotoxin may help to further elucidate the mechanism(s) of how CTL lyse tumor cells. (CS)