These are basic studies on the molecular mechanisms of lymphocyte mediated cell cytolysis in vitro. One of the mechanisms by which this has been proposed to occur is by a lymphocyte released material(s) termed lymphotoxin, which will cause lysis of cells in vitro. Recent studies indicate that while heterogenous, LT molecules from man and experimental animals comprise a system of noncovalent protein subunits which can associate together, and with antigen binding receptors, to form high molecular weight complexes. Functional studies of various human LT forms reveal the smaller forms are only weakly lytic or growth inhibitory to cells in vitro. In contrast, the larger forms, not containing antigen binding receptors, are highly lytic and capable of causing rapid nonspecific cell destruction. A fundamental new finding is that both human and murine LT forms have been identified from alloimmune cells that are associated with receptor(s) with specificity for target cells. These forms can cause rapid specific lysis of those target cells in vitro. We have found antibodies directed at the high MW LT forms of human LT are potent inhibitors of various forms of CML reactions in vitro. Additional studies with protease inhibitors reveal LT molecules require activation, perhaps by the target cell itself, before they become lytic. Employing a variety of modern biochemical techniques and utilizing various antisera already prepared against both human and murine lymphotoxins, we would propose to further separate and characterize the components of this system of cell lytic molecules in experimental animals and man. In addition, we will also study at the molecular level, the functional capacity of each of the various LT forms, and attempt to determine if these molecules are effectors in CML reactions in vitro.