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Two functional tissue culture cell lines, MTD and MTF cell lines, have been isolated from a mouse mammary tumor. MTD cells are epithelial and retain the ability to transport fluid leading to the formation of three-dimensional fluid-filled multicellular structures called "domes" or "hemicysts". Another property of MTD cells is the production of murine mammary tumor virus (MTV). Release of MTV into the culture medium was verified by immunological, electrophoretic and enzymatic analyses. Addition of dexamethasone in the culture medium enhanced both the formation of domes and the production of MTV. Thus, MTD cells retain the morphological and functional properties of the original mammary tumor cells.MTF cells show the fibroblastic morphology in subconfluent cultures. After reaching confluence, however, these cells gradually accumulated triglycerides in the cytoplasm and eventually assumed the morphology of fat cells. This adipose conversion was greatly enhanced by the presence of insulin in the culture medium. The morphological resemblance of adipose-converted MTF cells to the mammary fat cells suggests that the MTF cell line was derived from the mammary fat pad stroma. These functional cell lines will be useful to study cell differentiation as well as cell-tocell interactions in the mammary gland.Since the pioneering study of YASUMURA, SATO and co-workers (I), many in vitro cell lines retaining differentiated functions have been isolated and widely used for a variety of purposes. To establish in vitro model systems to study morphological and functional aspects of mammary gland cells, we isolated two cell lines which retained the original properties. Mouse mammary tumor cells in primary monolayer culture show the ability to transport fluid which leads to the formation of three-dimensional fluid-filled multicellular structures called "domes" (2). Another property of mouse mammary tumor cells is the production of murine mammary tumor virus (MTV) in response to glucocorticoid hormones (2-4). To isolate cell lines which retain both of these properties, we cloned the areas of epithelial cells engaged in dome-formation. The domeforming epithelia1 cell line that was finally isolated released abundant MTV into the culture medium.Mammary fat pad is the site for growth and morphogenesis of mammary epithelial cells (5). To understand the influence of mammary fat pad cells on mammary epithelial cells, in vitro cell culture systems have been strongly desired. For this purpose, we have established a preadipose cell line. The preadipose cells retained the ability to differentiate into fat cells which 223

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