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A total of 68 Swiss albino mice of either sex were thy; mectoraized at the following time intervals after birth: 22 during day one, 21 during day two, 17 on day four and 8 on day eight. Sixty-one nonoperated litter-rnates served as controls. All mice were given primary andtigenic stimulation with adsorbed tetanus toxoids when four weeks of age. Three weeks later a booster injection of fluid tetanus tokoid was given. With these time intervals thymectomized mice showed only slightly impaired primary responses but severely repressed secondary responses as compared to nonoperated litter-mates. This uncommon finding is difficult to explain at present and results are discussed with regard to other reports on the effect of postnatal thymectoniy on immune responses. Special emphasis is placed on the unknown effect of postthymectomy wasting syndrome. (auth)