The localization of actin was investigated in various organs of rat tissues; cerebellum, cerebrum, salivary gland, pancreas, stomach, intestine, kidney, spleen, and testis by means of light microscopic histochemistry using N- (7-dimethylamino-4-methylcoumarinyl) maleimide labeled heavy meromyosin (DACM-HMM). Positive staining of actin was observed in smooth muscle cells of all organs examined, and was present in the apical part and periphery in the epithelial cells of the salivary gland, pancreas, stomach, intestine and renal tubules. Actin was also present in myoepithelial cells of the salivary gland and exocrine pancreas. It was concluded that actin is generally present not only in smooth muscle cells but also in non-muscle cells of various organs, and our histochemical method is suggested to be specific for staining actin in cells compared with immunohistologic methods.