Signal transduction leading from receptor-ligand binding to any particular type of cellular activation appears to involve a number of biological steps. The term co-capping is used to describe lateral membrane protein interactions in which surface proteins are observed to redistribute together. After colchicine or vinblastine destroys the integrity of microtubules, strong ligand-receptor interactions appear to somehow overcome the microtubule inhibition of capping. During ligand-induced receptor capping, intermediate filaments have been shown to accumulate beneath capped structures using both electron microscopic and fluorescence staining techniques. Based primarily on biochemical and immunological criteria, intermediate filament proteins have been classified into five major groups: vimentin, keratin, desmin, glial filaments, and neurofilaments. During ligand-induced receptor capping, the myosin light chain kinaseis also preferentially accumulated under the surface cap-structure. Cytosolic free calcium, either entering through the plasma membrane or released from intracellular stores, has been proposed to be a regulator of a large number of cellular activities.