A total of 68 Swiss albino mice of either sex were thymectomized at the following time intervals after birth: 22 during day 1, 21 during day 2, 17 on day 4 and 8 on day 8. Sixty-one nonoperated littermates served as controls. All mice were given primary antigenic stimulation with adsorbed tetanus toxoid when 4 weeks of age. Three weeks later a booster injection of fluid tetanus toxoid was given. With these time intervals thymectomized mice showed only slightly impaired primary responses but severely repressed secondary responses as compared to nonoperated littermates. This uncommon finding is difficult to explain at present and results are discussed with regard to other reports on the effect of postnatal thymectomy on immune responses. Special emphasis is placed on the unknown effect of post-thymectomy wasting syndrome.