The contractile protein actin was demonstrated in formalin-fixed and paraffin-embedded human tissue by the peroxidase-antiperoxidase method of Sternberger. Both types of muscular cells and some other cell types showed positive staining. In skeletal muscle, the staining was seen diffusely in the cytoplasm with accentuation of cross-striations (I-bands). The cytoplasm of smooth muscle cells was also diffusely stained. Pericytes of blood vessels and myoepithelial cells of salivary glands and breast were clearly defined by positively stained smooth muscle cells and pericytes; this method proved superior to conventional elastic tissue staining for studying pathological conditions of blood vessels, such as vasculitides or invasion by neoplasms. Participation of myoepithelial cells in neoplasms of the breast and salivary glands could also be evaluated. This method can provide information about histogenesis and differentiation of a variety of soft-tissue neoplasms, in conjunction with other immunohistochemical methods for various tissue-specific markers.