Transformed fibroblasts had a low content of ATP when grown at a high cell density and a high content of ATP when grown at a low cell density. Concanavalin A agglutinated transformed cells with a low, but not those with a high, ATP content. Transformed cells with a high ATP content gained agglutinability after ATP depletion by inhibitors of the energy-generating systems, and those with a low ATP content lost their agglutinability after restoration of a high ATP content by glucose. Fixation of the surface membrane by formaldehyde, glutaraldehyde, or LaCl(3), inhibited agglutination of cells with an ATP content that allows agglutination. Normal fibroblasts grown at a high or a low cell density were not agglutinated by concanavalin A. Depletion of the cellular ATP content of normal cells induced agglutination only in cells grown at a high, but not at a low, cell density. A similar number of concanavalin A molecules was bound to the surface membrane of agglutinating and nonagglutinating fibroblasts. It is suggested that a high content of ATP inhibits the movement of concanavalin A binding sites, and that a low content of ATP allows, in transformed cells, a new distribution of binding sites to form the clusters required for cell agglutination. Agglutinability of transformed cells is determined by ATP content, and in normal cells changes in the content of ATP are by themselves not sufficient to induce agglutination. Transformed cells, therefore, do not have a control, presumably for membrane stability, that exists in normal cells.