We have established an explant-cell culture system for mammary gland tumors from c-myc oncogene-expresing trangenic mice and potentially other transgenic strains. By coating culture dish surfaces with fetal bovine serum and using culture media supplemented with low serum and growth factors, the mammary tumor specimens could be maintained in culture for over 3 mo. Throughout the culture period, the explants produced adundant outgrowths of epithelial cells. As the outgowths of epithelial cells filled the dishes, the explants were serially transferred from one dish to another—a process that could be repeated at least six times, thus providing a continuous supply of primary tumor cells. This culture system provides a useful tool for studying the biology of mouse mammary gland tumors and possibly tumors from other organ sites.