Neonatal guinea pigs during the first 2 weeks of life did not indicate the presence of delayed hypersensitivity intradermally, after sensitization with purified soluble antigens in dose levels that induced detectable delayed hypersensitivity in the skin of adults. Although Arthus type allergy was detectable in newborns, circulating antibody frequently preceded its appearance by several days. Passive Arthus reactions were not produced in newborns as readily as in adults. Contact hypersensitivity and allergic encephalomyelitis were induced in newborns, but corneal reactions were not. Total body irradiation with 200 r inhibited antibody formation in newborns, as in adults. In addition, the induction period for anamnestic responses in newborns and the antigen elimination rate were the same as in adults. Passive transfer of delayed hypersensitivity from sensitized newborns to normal adults was accomplished.