To investigate the effects of the T-cell deprivation on viral mammary tumorigenesis, two double congenic mouse strains of the DDD genetic background, DDD/1-Mtv-2/Mtv-2, nu/nu and DDD/1-Mtv-2/Mtv-2, nu/+, were produced by the cross between DDD/1-Mtv-2/Mtv-2 (DDD-Mtv-2) and DDD/1-nu/nu mice, followed by repeated intercross breedings. Expression of the mouse mammary tumor virus (MTV)-gp52 antigen was demonstrated in the mammary glands of mice from 14 days on, in both -nu/nu and -nu/+ females. Mammary gland development was comparable in both strains, but, the incidence of mammary cancer was lower in the T-cell-deprived mice.