Exposure to low environmental temperature caused a decrease in the half-life of human albumin (HA) in rabbits injected with 20 mg HA at birth, and a twofold increase in the proportion of animals which lost their tolerance by 150 days of age. Administration of thyroxin produced an even greater effect with respect to tolerance loss. Simlar mechanisms may be involved in the effects of cold exposure and thyroxin administration on tolerance duration. One possible mecahnism is that the duration of tolerance is dependent upon the metabolic half-life of the tolerance-inducing antigen. An alternative mechanism could be a cold- or thyroxin-induced enhancement of the recruitment of immunologically competent cells from an undifferentiated population of stem cells.