1. The effects of corticosterone and the time of its addition to cultures, on concanavalin A (Con-A) and pokeweed mitogen (PWM)-induced lymphocyte proliferation were studied. 2. Peripheral blood lymphocytes (PBL) were isolated from Cornell K-strain Single Comb White Leghorn immature male chickens and cultured with various concentrations of Con-A or PWM. Corticosterone, in different concentrations, was added to the cultures either 2 h before or 2 h after the addition of the respective mitogen. 3. Addition of corticosterone 2 h before the mitogens caused a significant suppression of lymphocyte proliferation in response to both Con-A and PWM stimulation. Also addition of corticosterone 2 h after the mitogens caused significant suppression of proliferation in response to both mitogens; however, the degree of suppression was not as great. 4. The results indicate that after early activation events are initiated by the mitogens, lymphocytes are less sensitive to the effects of corticosterone. Because less suppression was seen in the cultures preincubated with PWM than those with Con-A, it is likely that there are different sensitivities to corticosterone in the cell populations that respond to these mitogens.