The age dependent events influencing the rosette forming capacity of Balb/c thymus and spleen cells against autologous or syngeneic erythrocytes were examined. A large number of autologous and syngeneic rosette forming cells (RFC) were observed in normal Balb/c mice in vitro. RFC were significantly greater in the thymus than in the spleen. The rosette forming T-cells (T-RFC) have the following characteristics: newborn Balb/c thymus has T-cells which react syngeneic erythrocytes from older donors. The T-RFC showed broad cross-reactivity with erythrocytes from other mouse strains but low reactivity with human or sheep erythrocytes. The auto and syngeneic T-RFC could be enhanced by non-specific serum proteins (FCS or BSA) or EDTA but was effectively inhibited by normal mouse serum. T-RFC resided in the cortisone sensitive population. The data indicate that the development of autologous and syngeneic rosette formation of thymus cells is dependent on the age of the erythrocyte donor. The age dependent change on the erythrocyte surface occurred relatively early in the life of the animal. The results also imply that certain subpopulations of thymus lymphocytes are capable of recognizing possible surface antigenic changes of the erythrocytes.