Numerous studies have shown the proliferative response to various mitogenic stimuli of peripheral blood lymphocytes from elderly humans to be impaired. The present investigation examined the termination phase of antigen-stimulated proliferative responses of lymphocytes from elderly and young subjects. In both groups, the responding lymphocytes appeared to stop proliferating and enter the resting stage of the cell cycle when a certain total number of progeny had been generated, suggesting the phenomenon of density-dependent regulation of growth. Lymphocytes from elderly subjects stopped proliferating when significantly fewer progeny had been generated than did those from young subjects. The data suggest, therefore, that lymphocytes from elderly humans may have increased sensitivity to one or more of the factors which cause density-dependent inhibition of growth.