Actin has been specifically detected in the intercellular matrix of mouse smooth muscle cells and along the vascular internal elastic membrane by means of immunoferritin-electron microscopy, employing human antibodies to smooth muscle actin. The presence of actin in the smooth muscle matrix suggests that this protein may have relevance in controlling cell-to-cell adhesion and the sliding of one smooth muscle cell over the other, both in contracted and expanded status. The association of actin with the elastic membrane may represent the anatomical basis of a functional link between elastic membrane, adjacent fibronectin and smooth muscle cells. Thus the transmission of movement from the smooth muscle cells to the elastic membrane can be achieved.