The induction of delayed type of hypersensitivity to diphtheria toxoid in the guinea pig was not inhibited by total body irradiation up to 300 r in intensity. X-ray doses of 200 to 300 r administered about 18 hours before sensitization caused the period of delayed hypersensitivity to be extended to the 19th to 21st day postsensitization in. the absence of circulating antibody. X-ray doses of 50 to 100 r caused a decrease in the titer of circulating antibody, although delayed hypersensitivity lasted for a normal time. When 300 r irradiation was administered 18 hours after sensitization, delayed hypersensitivity lasted for the usual period and circulating antibody first appeared at the usual 13 to 14 days after sensitization. Introduction of normal serum or leucocytes into irradiated animals apparently did not reduce damage to the mechanism regulating the rate of antibody synthesis.

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