A classification of mouse mammary tumors in Dunn's miscellaneous group including recently reported types is presented. Adenocarcinoma type Y, first reported in (C3H X Y)F1 mice, is characterized by tubules that branch at acute angles and are lined by cuboidal epithelium. Adenocarcinoma type L is characterized by alcelike appearance and has projections of tumor cells with cytoplasmic vacuoles into lumina. Undifferentiated carcinomas in strains SWR/J and BABL/c contain sheetlike masses of epithelial cells with very little tendency to form glands but which may contain foci of cells with keratohyaline granules. Pale cell carcinomas, which occur in strain GR mice, are hormonally responsive and are characterized by the presence of pale cells and cystic spaces lined by basophilic cells. Foci of cells with acidophilic hyaline bodies and keratohyaline granules and with intercellular bridges distinguish this neoplasm from adenocarcinoma type B. A second hormonally responsive mammary tumor in strain GR mice is adenocarcinoma type P, which consists mainly of acini lined by a single layer of polyhedral cells. Carcinosarcomas of BALB/c mice have a myxoid stoma and, also containing glands, resemble mixed mammary tumors of the bitch. Criteria for differentiating trichoepitheliomas from mammary tumors are discussed.