Mammary tumor virus (MTV) antigens in both sexes of GRS/A, SHN and C₃H mice were examined in the sweat and sebaceous glands by immunoperoxidase technique using antiserum against gp52, envelope protein, or p27, core protein. Balb/c mice were used for reciprocal foster nursings with these inbreds to discriminate the expression of endogenous MTV from that of exogenous MTV. Both antigens were first detected around the age of 4 months in the sweat glands of mice with endogenous GR- or SHN- MTV. A linear staining of gp52 was seen along the luminal borders of glandular cells, and the reaction products for gp52 were demonstrated on the apical cell membranes, where no virion could be seen ultrastructurally. A diffuse staining of p27 was found in the cytoplasm of some glandular cells, where MTV particles could not be detected. In the sebaceous gland of the same mice, however, only p27 was first detected at the age of 60 days. A dot-like staining of p27 was found in the perinuclear region of some glandular cells, where an aggregation of intracytoplasmic A particles could be seen under an electron microscope. These positive stainings were unrelated to sex. In such skin appendages of all examined C₃H mice and Balb/c mice with GR- or SHN- MTV, no antigen expression could be seen up to the age of 500 days. Therefore, some genes might be able to regulate the expression of endogenous MTV antigens in the skin appendages, while their glandular cells would have no receptor for exogenous MTV, namely the so-called“milk factor”.