Cytoskeletal proteins are being studied to understand how these proteins interact to produce the various motile activities of cells. The interaction of cytoplasmic myosin with actin filaments and the hydrolysis of ATP by myosin provide the force that drives these processes. The interactions of cytoplasmic myosins with actin are regulated by specific calcium calmodulin dependent kinases. Unlike with smooth muscle myosin, there is a linear relationship between the level of cytoplasmic (thymus) myosin phosphorylation and stimulation of the actin-activated ATPase of this myosin. Thus, even low levels of phosphorylation can stimulate motile activity. Turbidity, ultracentrifugation, and electron microscopy were used to examine the equilibrium between myosin filaments and myosin monomers or small oligomers. This equilibrium is dependent on ionic strength, divalent cation concentration, type of anion used, and on whether the myosin is phosphorylated. While phosphorylation promotes filament formation, it appears unlikely that this is the principal mechanism for regulating the participation of cytoplasmic myosins in force development. Fodrin or brain spectrin is a calmodulin binding protein that appears to link the cytoskeleton to the cell membrane. Under approximately physiological conditions, fodrin inhibits the actin-activated ATPase of myosin. More fodrin is required for inhibition in the presence of calcium than in its absence, but calmodulin has no effect on this inhibition. Thus, in the region of the cell where fodrin is localized, the interaction of myosin with actin is inhibited. The calcium sensitivity observed in vitro provides a potential mechanism for regulating this inhibition. Detergent treated T-lymphoma cells are being used as a model system for examining the role of cytoskeletal proteins. While these cells are permeable to large proteins, i.e. antibodies and myosin light chain kinase, their cell surface proteins still cap in response to concanavalin A binding. This capping requires calcium and ATP.