AbstractThymus cells from neonatal and infant mice were found to have a high capacity to prevent mortality from acute graft‐vs.‐host disease as compared with spleen cells from stable radiation chimeras. This suppressive capacity of thymocytes decreases with age after birth as was demonstrated by semi‐quantitative cell titrations. This suppressor activity is restricted to syngeneity of the graft‐vs.‐host disease‐including cells. The thymic suppressor cells are Thy‐1+and Lyt‐1+and IgG−and IgM−. They do not agglutinate with peanut agglutinin and have a high electrophoretic mobility.In vitroirradiation experiments showed that the suppressor cells are radiation sensitive. These results are compared with the available information on cells suppressing delayed‐type hypersensitivity reactions and those suppres