Nine epithelial cell strains were established from normal mouse liver, mammary gland, ovary, and ear skin. After overnight digestion by collagenase in growth medium, intact glandular fragments produced many islands of epithelial outgrowth when plated in an enriched medium. Contaminating fibroblasts were removed by selective trypsinization. These strains continue to show morphologic and ultrastructural features typical of epithelial cells after 10-50 subcultures. In addition, several strains continue to form secretory vesicles, or hemicysts, on confluent cell sheets. All but 1 strain have maintained a majority population of diploid cells, but all strains show increasing numbers of near-triploid cells. Five strains produced tumors (benign cystadenomas, adenocarcinomas, and sarcomas) when inoculated into isogeneic mice. Type-C oncornaviruses were seen in all but 2 strains.