A variant subpopulation of C57BL/6 mouse fibrosarcoma cells that had very low tumorigenic potential was isolated from a highly tumorigenic parent fibrosarcoma cell culture. The adhesive characteristics of parent cells and variant cells were compared. The low-tumorigenic variant cells were released from the surfaces of plastic dishes, from protein-coated dishes, or from monolayers of fibroblasts or endothelial cells by protease treatment much more readily than were the parent cells. There was no difference between the variant cells and the parent cells in EDTA sensitivity or sensitivity to mechanical agitation under the conditions used. Also, no difference existed between the variant cells and the parent cells in rates of attachment to the surfaces of plastic dishes or to monolayers of endothelial cells. The variant cells were characterized by high levels of chymotrypsin-like esterase activity (two to three times increased over parent cell levels), but there was only a slight difference between the variant cells and the parent cells in caseinolytic or fibrinolytic activity.