We have previously demonstrated that life span and immune reactivity can be altered by dietary manipulation. Dietary manipulation has also been shown to affect tumor production in cancer prone strains of mice and to alter the course of antibody production in autoimmune reactive strains of mice. It has not become our goal to define the effect of dietary manipulation on cells capable of regulating immune responsiveness in long- and short-lived strains, and in cancer prone and autoimmune strains. We will explore the significance of our recent finding that fetal liver cells are immunodepressive, utilizing transfer of fetal liver cells and thymus following thymectomy to alter the course of autoimmune reactivity, and to increase life span beyond normal limits in an already long-lived mouse strain. We will also test by classical genetics methods whether H-2b heterozygosity has the ability to prolong immune vigor and natural killer cell reactivity, and whether H-2b heterozygosity is significant in producting longevity. It is the goal of these studies to both explore immune factors which confer longevity, and to test factors which may alter normal or abnormal immune reactivity late in the life span.