The lymphocyte subpopulations that are activated by 2-ME, LPS, poly IC, and PPD were studied in terms of their maturational characteristics. Attempts to stimulate hepatic and splenic lymphoid cells from mice of different ages with these mitogens demonstrated a well ordered sequence for the emergence of mitogen responsiveness in C3H mice: reactivity to LPS and Poly IC was observed early in maturation and was followed by that to PPD, and finally by the development of responsiveness to 2-ME. The same sequence appeared when the mitogen responsiveness of lethally irradiated, fetal liver-reconstituted syngeneic adult recipients was examined. The mitogenic action of 2-ME was dissociated from its ability to enhance lymphocyte reactivity to other mitogens in mice too young to respond to 2-ME as a mitogen. Experiments in which additivity of responses was assayed by adding mitogens to culture singly or conjointly indicated that LPS and Poly IC activate nearly identical B lymphocyte subpopulations, whereas PPD stimulates a subset of cells distinct from that which is responsive to the former two mitogens. The mitogen responsiveness of CBA/N mice, relative to normal CBA/WEHI mice, was shown to decrease as a function of the maturity of the subpopulation of lymphocytes activated. The CBA/N mouse was shown to be unresponsive to stimulation by 2-ME.