Vascular musculature was studied in cerebral arteriovenous malformations using a monoclonal antibody against the muscle protein actin in 20 cases. The more typical vessels (arterial and venous types) and a number of abnormalities of the muscular layer were identified. The latter included (1) partially developed media; (2) two layers of the media separated by a well-formed internal elastic membrane; (3) total or partial disarray of the muscle coat; and (4) partial absence of the media. Previously described large capillaries proved to be postcapillary venules by virtue of having a distinct muscular layer. Serial sectioning indicated that the previously described "polypoid projections" of the media are mostly artifacts and the concept of "arterialization of veins in arteriovenous malformations" could not be substantiated. The actin method proved to be a useful adjunct to the conventional stains for accurate and selective detection of smooth-muscle cells.