Thymocytes from BALB/c mice ranging in age from 16 days of gestation to adulthood were stimulated with the phytomitogens phytohemagglutinin (PHA), concanavalin A (Con A), and pokeweed mitogen (PWM), or with syngeneic or allogeneic adult spleen cells in the mixed lymphocyte reaction (MLR). Fetal thymocytes responded well to PHA and to allogeneic cells in the MLR, but poorly to Con A and PWM. Neonatal cells showed a sharp decline in PHA and MLR responsiveness and a concomitant increase in Con A and PWM reactivity. The syngeneic MLR was maximal during this period. Adult thymus cells responded poorly to PHA and not at all to syngeneic cells, but showed much increased responsiveness to Con A and PWM. These results are discussed with regard to a model proposing two different but interacting lines of thymus lymphocyte differentiation.