A method has been developed for the study of the forces which individual cells exert during their locomotion. Polydimethyl-siloxane (silicone fluid) was crosslinked on its surface by brief flaming to form a thin layer of silicone rubber. Tissue culture cells of many types were then plated out onto these rubber substrata and the propulsive forces these cells exert as they adhere and spread became visible as wrinkles and other distortions in the rubber. From time-lapse films of these distortions, it appears that the component cells of the body move by exerting shearing forces through their plasma membranes. How these forces are exerted and how this technique for observing them could be made more quantitative are discussed.