Lymphocytes stimulated in mixed leukocyte cultures and left for 13-17 days, i.e. beyond their peak proliferative and cytotoxic reactivities, can be restimulated to give a secondary-type rapid and strong proliferative and cytotoxic response when confronted with cells of the original sensitizing cell donor. We have concerned ourselves primarily with the requirements of restimulation for the presence of LD and/or SD stimuli on the restimulating cells. (a) The low level cell-mediated lympholysis (CML) associated with LD differences in a primary CML can be restimulated to give a secondary-type response by those same LD antigens. (b) If the original sensitizing cells differ from the responding cells by both LD and SD antigens, restimulation with only the LD antigens, or third-party cells presumably carrying cross-reactive LD antigens, can restimulate the secondary CML responses directed against the SD antigens on the original sensitizing cells. (c) The presence of SD antigens on the restimulating cells that are cross-reactive with the primary sensitizing SD antigens (as determined in a primary CML) leads to the preferential activation of cytotoxic T lymphocytes reactive to those antigens although maximum cytotoxicity is still directed at cells carrying the original sensitizing SD antigens. A model to explain these results is presented.