Brain calmodulin-dependent protein kinase was isolated as the major cytoskeletal component of the post-synaptosomal fraction, accounting for about 50% of the total protein. The alpha- and beta-subunits of the enzyme were present in a ratio of about 6:1. The cytoskeletal preparation was also distinguished by the regular presence of tubulin and actin in approximately equimolar amounts to the beta-subunit. Electron microscopic morphology suggested that the enzyme formed a hierarchy of cytoskeletal complexes with an average diameter of 85 nm. These results indicate that cytoskeletal calmodulin-dependent protein kinase may have a structural as well as a catalytic role in neurons. The cytoskeletal enzyme could be solubilized in 8 M urea, displaying similar properties to its cytosolic counterpart. The postsynaptosomal cytoskeleton provides a simplified and well-defined model for the study of the protein-protein interactions involving calmodulin-dependent protein kinase.