A variety of epithelial cells and fibroblasts fail to move over one another's upper surfaces in culture, resulting in monolayering. The failure of seeded fibroblasts to adhere to and spread on epithelial cell surfaces suggests that monolayering in culture is due to the lack of adhesion of the upper cell surface, at least of epithelial cells. Seeded fibroblasts and postmitotic, rounded fibroblasts likewise fail to spread on the upper surfaces of spread fibroblasts, suggesting that the inability of the upper cell surface to support spreading may be a general phenomenon. Inert particles and cell processes do not adhere directly to the upper cell surface. However, they can initiate adhesions to the surface at a cell's free margin, suggesting a variation of adhesive properties over a cell's surface.