Depression of weak allograft immunity in the mouse by neonatal or adult exposure to urethan.

Treatment of adult mice of three strains with mini mally carcinogenic doses of urethan caused an impair ment of cellular immunity as measured by prolongation of allograft survival. This effect was only detectable at the highest dose of urethan tested (150 mg/mouse) and was confined to the weaker strain combinations examined for allograft prolongation. Newborns appeared to be more susceptible to the depressive effect of urethan than did adults. In the BALB/c strain, doses of 120 and 150 mg urethan administered to adults were ineffectual in pro longing allograft survival. However, a dose of 1.0 mg ad ministered to 4-day-old males (but not females) signifi cantly prolonged allograft survival in some individuals when the mice were tested at 3 months of age. These facts are discussed in light of the pronounced carcino genic effect of urethan in young and newborn mice.