Seven weeks after orchiectomy, 19 and 10 month old mice had mean thymic weight increases of 34 and 128%, respectively, compared to sham-operated controls. Peripheral WBC counts were greater (p<.05) in castrates of both age groups. On a cell for cell basis, the mitogen response of castrate spleen cells was not significantly greater than controls, and in some instances was significantly less. Although individual cell function may not be augmented after castration, in vivo immune response may be potentiated due to an increase in absolute number of lymphoid cells. © 1984 Taylor & Francis Group, LLC.