We report the immunological characterization of the subunit of the intermediate sized (100 A) filaments from muscle cells. The protein as isolated from smooth muscle (chicken gizzard) has an apparent molecular weight of 50,000. It is insoluble in buffers that solubilize myosin and the majority of actin, but becomes soluble in the presence of urea. Under a variety of experimental conditions, that include the presence of 8 M urea, this new protein comigrates with actin during purification studies. The two proteins can be separated from each other by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, and antibodies have been elicited against the 50,000 dalton protein purified by using this technique. These antibodies crossreact with the partially purified protein in urea, but show no detectable cross reaction with actin or myosin. Indirect immunofluorescence reveals that in skeletal muscle this protein is found in close association with the Z lines of the sarcomeres and extends between the Z lines of adjacent myofibrils; it is also associated with filamentous structures that run along the length of a muscle fiber both in close association with the plasma membrane and between myofibrils at the level of their Z lines. In heart muscle, the protein shows the same distribution as in skeletal muscle. In addition, it is found intimately associated with intercalated disks and areas of membrane interaction between laterally associated heart muscle cells. The immunofluorescent localization to the subunit of the 100 A filaments suggests that in muscle cells this molecule may serve to link actin filaments at the level of the Z line (or intercalated disk) with the muscle plasma membrane. We believe that it functions in muscle primarily as a three dimensional matrix which interconnects individual myofibrils to one another and to the plasma membrane at the level of their Z lines. In this manner, this molecule may provide a framework that mechanically integrates all the contractile myofilaments during the contraction and relaxation of muscle. As a means of indicating its linking role in muscle, we have termed the protein desmin (from the Greek delta epsilon sigma mu os = link, bond).