Localization of actin-like protein in the mouse brain was studied by the immunofluorescence method using FITC-labeled antiactin antibody. Antibody against actin was prepared according to Lazarides (1974). The problem of poor antigenicity and the presence of contaminants in the antigen preparation was circumvented by way of denaturation of the protein with sodium dodecyl sulfate (SDS) and subsequent purification of the antigen by SDS polyacrylamide gel electrophoresis (Lazarides, 1974). Intense specific fluorescence against actin was noted in the cytoplasma of the epithelial cells of the choroid plexus, of the ependymal cells of the ventricles, and a little less one in arachnoidea and in leptomeninges, as well as in the muscle layers, endothelial linings, and adventitia of the arteries. Diffuse but weak fluorescence was noted also in the neuropil of the grey matter of the brain. Among the latter, fluorescence in the granular layer of Ammon's horn and of the cerebellum appeared clearer than the diffuse one in the rest of the grey matter. Functional significance of the presence of actin-like protein in the above tissues was discussed.