The results of explantation of 167 human epithelial tumors from various sites are described. Growth of specific tumor cells was noted in half the explants. Three stable lines of cells were obtained from tumors of the pancreas (CaPa strain), stomach (CaVe strain), and mammary gland (CaMa strain). Growth of homogeneous epithelium-like cells was observed in cultures of the CaPa strain from the time cultivation was started. During more than 3 years of cultivation the cell composition of the strain did not change. Fibroblast-like cells grew in the cultures of the CaVe and CaMa strains during the first months of cultivation. These cells were later displaced by emergent epithelioid cells. In the next 3 or more years of cultivation the cells of this strain retained their homogeneous epithelioid appearance. Cells with a subtriploid chromosome number predominated in the cultures of the CaMa strain. The cells of the CaMa strain had the properties of epithelium when grown in a plasma clot and sponge substrate, and also when implanted in hamsters.