Four experiments were conducted to determine possible differences in the in vitro concanavalin A (ConA) response between two lines selected either for high (H) or low (L) antibody response 5 d after intramuscular immunization with SRBC. In all four experiments, the cell proliferation after stimulation with ConA was higher, although not always significantly so, in the L line than in the H line, independently of dose of Con A and source of lymphocytes. It can be concluded that selection for anti-SRBC antibody response affected the cellular response in chickens. Previously reported results, in other chicken lines selected for humoral response to SRBC after intravenous immunization with SRBC, showed an opposite line difference in mitogen response. These opposite results point to the fact that comparable selection protocols for immunological variables do not necessarily have a comparable influence on the diverse components of the immune response.