The effects of age on the immune response were studied in highly responding and poorly responding strains of inbred rats. Changes in the immune response associated with age were found only in the highly responding strain: the antibody response fluctuated up to 10–12 weeks of age, showed no significant change up to 60 weeks of age and decreased thereafter. In addition, immunization increased the weight and the number of germinal centers in the spleen and stimulated cortical nuclear phagocytosis in the thymus. The factors influencing antibody formation at different ages appear to modulate the genetically set capability of the immune system, and they are most conspicious early and late in lif