The present study describes changes which occur in the lymph nodes, spleen, and thymus lymphocytes of aging AKR/J mice. Lymphocytes from mice between 3 and 14 months of age were investigated with respect to three thymus-dependent properties, i.e., in vivo homing to lymph nodes, responsiveness to phytohemagglutinin, and expression of the θ AKR antigen. Decreases in the numbers of lymph node-seeking, recirculating lymphocytes in lymph node, spleen, and thymus, and diminished responsiveness of spleen cells to stimulation by phytohemagglutinin were observed with increasing age. No change in the sensitivity of lymph node cells to anti-θ serum was noted in the aging AKR/J mice. The significance of the decline in the size of the thymus-dependent recirculating lymphocyte pool with age is considered in relation to the concomitantly occurring onset of spontaneous lymphocytic leukemia, which is characteristic for the AKR/J strain.