Lymphocytes from C3H x CBA F1 mice, lacking detectable Mls-antigens, can specifically inactivate T-cell clones from the H-2-identical strain CBA, which are reactive against the C3H-determined Mls-antigen. Such F1 lymphocytes also proliferate in the spleens of irradiated CBA mice. The age dependence of the mixed lymphocyte culture stimulatory capacity and the capacity to proliferate in irradiated CBA recipients of lymphocytes from C3H x CBA F1 mice were compared. It was observed that appearance of Mls-bearing cells is preceded by the appearance of F1 cells with strong proliferative capacity in irradiated CBA recipients. The in vivo proliferative response is also dependent on the age of the recipient. The varying response is not reflected in the quantity of anti-C3H Mls-reactive cells in CBA mice. A strong linkage between the genes controlling the above proliferative and stimulatory capacities of C3H x CBA lymphocytes was found.