Nine epithelial cell strains were initiated from mouse liver, ovary, ear skin, and mammary gland. Clusters of epithelial cells were liberated from glandular tissues by collagenase digestion, and fibroblasts were selectively removed from culture flasks by a series of brief exposures to trypsin: Versene. All strains maintained the morphology and growth pattern typical of epithelial cells cultured for many generations. Several strains of liver or mammary gland origin continued to produce secretory vesicles in vitro and form well-differentiated glandular or cystic tumors when inoculated into newborn isogeneic mice.