The distribution of mouse mammary tumor virus (MuMTV) antigens was studied in normal, preneoplastic, and neoplastic mammary epithelia. from female BALB/cfC3H mice with the use of a polyvalent anti-MuMTV serum and indirect immunoperoxidase techniques. The MuMTV antigens were on the apical surface or in focal cytoplasmic aggregates or diffused throughout the infected cells. As much as 70% of all cells in adenocarcinomas and as much as 100% of all cells in preneoplastic hyperplastic alveolar nodules contained MuMTV antigens. Comparable percentages of cells from mammary glands of multiparous mice were MuMTV-positive. Some mammary tissues of nulliparous and primiparous mice did not contain detectable MuMTV antigen. The MuMTV antigen-containing cells in lactating mammary glands tended to be in discrete lobuloalveolar clusters surrounded by antigen-negative alveoli. The percentage of MuMTV-positive cells in a given gland was proportional to the amount of virus found in the animal's milk.