1. The cytoplasm of cells is a colloidal network of contractile proteins. Actin filaments are the major components of this network. Other contractile proteins interact with these filaments to create structural rigidity and movement. 2. The structure and function of contractile proteins is striated muscles is well characterized and thus provides a good example for extrapolitation to an analysis of contractile-protein structure and function of nonmuscle cells. However, the interaction of contractile proteins of various cells may be unique. 3. The study of contractile proteins in cells other than muscle has distinct difficulties: (a) The proteins are present in much lower concentration than in muscle, and only a few cell types are obtainable for study in quantities comparable to muscle. (b) Proteolysis and other detriments may be more severe in nonmuscle cells. (c) The organization of contractile proteins is difficult to define in nonmuscle cells. (d) The effort is diffuse; investigations examine a wide variety of different, or less commonly the same, cells. Reflecting these difficulties, a catalogue of putative nonmuscle cell fucntions related to contractile proteins reveals no one system to be definitively worked out. Nevertheless, the ubiquity of contractile proteins and the obvious importance of their intractions presages increasing relevancy for physiology and medicine.