Actin is one of the major protein constituents of the postsynaptic density (PSD), a characteristic structural entity subjacent to the postsynaptic membrane in excitatory synapses of the vertebrate central nervous system. In isolated purified PSD preparations, it is present to the extent of 29 +/- 2 micrograms/mg of total protein, 90% of which is in the filamentous (F-actin) form. Iodination by a discriminatory labeling technique demonstrates that actin is located on the surface of the PSD from which it can be stripped by treatment with a mixture of strong anionic detergents, leaving behind an insoluble core held together by disulfide bridges, consisting in part of tubulin and "PSD protein".