Some interpretational problems associated with the myofibroblast, which affect how this cell is identified, are discussed. Questions addressed include distinguishing between "external" lamina ("basement membrane") and the fibronectin fibril of the fibronexus; the nature of stress fibers (bundles of smooth-muscle myofilaments with focal densities); the utility of some of these features to distinguish between myofibroblastic and smooth-muscle cell surfaces; and cytoskeletal immunophenotype. The following points are emphasized. Myofibroblasts have a surface characterized by prominent fibronectin fibrils and fibronexus junctions, which are distinct from lamina ("basement membrane"). This can permit a distinction to be made between smooth-muscle and myofibroblastic lesions and tumors. Myofibroblasts are typically positive for vimentin and alpha-smooth-muscle actin, but desmin is not a useful discriminant between smooth-muscle and myofibroblastic lesions. The main features for defining the myofibroblast are abundant rough endoplasmic reticulum; modestly developed peripheral myofilaments with focal densities (stress fibers); fibronexus junctions; vimentin and smooth-muscle actin immunostaining. Other features include a Golgi apparatus and collagen secretion granules, gap junctions, and actin-associated nondesmosomal junctions. Illustrations of the usefulness of these criteria in the diagnosis of soft-tissue lesions (myofibrosarcoma, so-called myofibroblastoma) are given.