Density-dependent inhibition of growth, plating efficiency on confluent monolayers of 3T3 cells, and growth in agar have been measured in epithelial tumor cell lines to determine whether they have properties in common with transformed mesenchymal cells. Five lines (RT4, RT112, J82, T24, and EJ) were derived from different human bladder tumors and HT29 was from a human colon tumor. All the lines resembled transformed "fibroblasts" in the absence of density-dependent inhibition of growth and the cell-surface large external transformation-sensitive protein, and they could form colonies on 3T3 monolayers. Only RT112, EJ, and HT29 were tumorigenic in nude mice, and the tumors had many of the structural and ultrastructural features of both the original tumor and the tissue of origin, even though the cells had been through many in vitro passages. Four of the lines (J82, T24, EJ, HT29) grew in agar, so that in some cell lines no correlation of growth in agar with tumorigenicity in nude mice was found: J82 and T24 were nontumorigenic and grew in agar, whereas tumorigenic line RT112 did not grow in agar. The ability to grow in agar did not appear associated with the production of high levels of plasminogen activators.