Monoclonal antibody to rat brain actin was easily produced using HVJ (Sendai Virus) M protein to enhance the antigenicity of the actin. This monoclonal antibody was determined to be IgM with a kappa light chain. By immunoblot analysis the antibody was also shown to react with rat brain actin but not with HVJ M protein on nitrocellulose sheets. Utilizing the antibody, neuronal cytoplasm in the cerebral cortex, the anterior and posterior horns in the spinal cord, the spinal ganglion and astrocytes showed positive immunohistochemical staining by light microscopy. However, Purkinje cells showed variable staining, some staining intensely, while others were negative. All of neurons in specific anatomical locations showed always positive staining but variable intensities. Vascular walls were stained only faintly. By electron microscopy, neuronal cytoplasm showed diffuse positive staining. Other areas showed a positive reaction, including dendrites, the postsynaptic densities, and a few capillary endothelial cells and arterial smooth muscle cells. The results suggest that the HVJ M protein was effective for producing monoclonal antibody to brain actin, and that the antibody could be utilized for the immunohistochemical study of neuronal elements in both normal and pathological conditions.