Rats thymectomized at birth gained weight and otherwise developed normally, but were found to be very susceptible to intercurrent infections. Both Arthus reactivity and delayed hypersensitivity to BSA were markedly impaired in rats thymectomized during the first week of life and significantly impaired in rats thymectomized as late as 3 weeks after birth. The inhibition of Arthus reactivity in thymectomized rats was well correlated with their failure to develop significant titers of precipitating or hemagglutinating antibody. However, natural heteroagglutinin titers were not altered in these animals, and no abnormality of serum proteins, including γ-globulin could be detected by paper electrophoresis. The loss of immunologic activity could not be corrected by injecting homogenates of spleen or thymus before and during the sensitization period. Splenectomy at birth did not influence Arthus or delayed reactivity.

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