We have reported that the serologic reactivity of young guinea pigs to mycobacterial antigens decreased proportionately as the sensitizing dose of heat-killed tubercle bacilli was increased (1). Using animals from 0 to 4 weeks old, we found that approximately 67% of those that received doses of 0.5 mg (dry weight) or less exhibited precipitating and complement-fixing antibodies against an unheated mycobacterial culture filtrate when tested 4 weeks after injection, whereas only 34% of the animals that received doses greater than 0.5 mg reacted serologically when similarly tested. Serologic unresponsiveness was induced quite effectively by injecting newborn animals with a 5-mg dose; only 1 of 14 animals sensitized in this manner exhibited anti-mycobacterial precipitins when tested 4 to 5 weeks later. Hypersensitivity to tuberculin purified protein derivative (PPD) in the serologically-unresponsive animals, however, appeared to be unaffected. Animals injected with the 5-mg dose at birth exhibited unequivocal skin reactivity to PPD 5 weeks later, but the reactions were smaller than those of adults that received the same sensitizing dose (1).