A method is described for measuring cell-mediated cytotoxicity, based on the incorporation of labeled leucine into actively synthesized proteins in viable target cells which have survived interaction with effector lymphocytes. The method was studied with in vitro or in vivo sensitized lymphocytes in xenogeneic or allogeneic systems. This method was found to be applicable to quantitative determination of cell-mediated cytotoxicity by in vitro sensitized lymphocytes against a syngeneic tumor.