Some interpretational problems associated with the myo broblast, which affect how this cell is identi ed, are discussed. Questions addressed include distinguishing between``externalīī lamina (``basement membraneīī) and the bronectin bril of the bronexus; the nature of stress bers (bundles of smooth-muscle myo laments with focal densities); the utility of some of these features to distinguish between myo broblastic and smooth-muscle cell surfaces; and cytoskeletal immunophenotype. The following points are emphasized. Myo broblasts have a surface characterized by prominent bronectin brils and bronexus junctions, which are distinct from lamina (``basement membraneīī). This can permit a distinction to be made between smooth-muscle and myo broblastic lesions and tumors. Myo broblasts are typically positive for vimentin and a-smooth-muscle actin, but desmin is not a useful discriminant between smooth-muscle and myo broblastic lesions. The main features for de ning the myo broblast are abundant rough endoplasmic reticulum; modestly developed peripheral myo laments with focal densities (stress bers); bronexus 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 (myo brosarcoma, so-called myo broblastoma) are given.