Devise a medium which will support the growth of normal mammary epithelial cells by proceeding in the stepwise manner of first finding the exact requirements of established tumor lines and then primary tumor cultures and finally the requirements of normal epithelia.  The notion is that established tumor lines have lost some of the requirements of normal cells.  In addition, we think that anything required by established tumor cells will be required by primary tumor cells and normal epithelia.  We also think that by proceeding with defined, serum-less media, we will eliminate inhibitory factors which have hampered attempts to culture normal epithelia.  Finally, in using serum-free media for primary tumor cells and normal tissue, the fibroblasts will die after two to three days so that only the growth requirements of epithelial cells are studies.