Delayed hypersensitivity to soluble purified protein develops after injection of antigen into the foot-pads of adult guinea pigs.This antigen may be administered (a) in a specific precipitate in antibody excess (1), (b) in minute quantities (2), or (c) as part of a conjugated protein (3-5). Ultimately, the delayed hypersensitivity may be followed by the appearance of detectable quantities of circulating antibody and the presence of Arthus type hypersensitivity. Formation of circulating antibody has been studied in newborn animals and found deficient when compared to adults (6-10). The presence of delayed hypersensitivity, on the other hand, has not been investigated extensively in newborns.Neonatal guinea pigs sensitized subcutaneously with live tubercle bacilli showed slight, if any, skin reaction to the injection of a watery extract of Mycobacterium tuberculosis despite the presence of gross tuberculous lesions in the regional lymph nodes and spleen (11, 12). The mother pigs under the same conditions showed at the skin-testing site marked erythema, edema, and necrosis. These neonatal animals were approximately as sensitive to the systemic toxic action of old tuberculin injected into the peritoneal cavity as were adult tuberculous guinea pigs. Similar results were described (13) after intraperitoneal sensitization with old tuberculin and skin testing subsequently with the same antigen. Uhr (14), however, demonstrated delayed allergy in newborn guinea pigs 7 to 10 days after foot-pad injection of ovalbumin in complete Freund's adjuvant. The passive transfer of cells from sensitized adults failed to induce delayed hypersensitivity in neonatal guinea pigs, rabbits, and humans (15-17). In contrast, one report (18) describes the passive transfer of tuberculin hypersensitivity after exchange transfusion of infants with blood from tuberculin-positive adults. Contact hypersensitivity, a probable variant of delayed hypersensitivity, has been produced in neonatal infants (19,20). In view of the evidence that newborn animals respond less readily than adults to antigenic stimuli, studies were initiated to determine the ability of neonatal guinea pigs to manifest delayed hypersensitivity under a variety of test conditions.In this paper, results are presented indicating that newborn guinea pigs can develop delayed hypersensitivity, but do not manifest it in the skin after intradermal injection of antigen. 707 on