Single cells displaying many characteristics in common with smooth muscle are now being identified in many organs. Although their origin remains elusive, they are nonetheless known to play a major role in fibroproliferative diseases and stromal reactions by virtue of their contractile properties. We have investigated the contractile properties and expression of smooth muscle-associated proteins in such a cell line derived from human foetal lung (IMR-90). For almost two decades, these cells have served as model fibroblasts in a wide variety of studies. And yet, IMR-90 cells manifest many features characteristic of differentiated smooth muscle cells: they exhibit the same elongated, slender form and the strikingly regular, longitudinal alignment of their actin-attachment sites, which are indispensable for coordinated contraction. Moreover, these adhaerens junctions also contain metavinculin, the smooth muscle analogue of vinculin, the stable expression of which has not previously been demonstrated in cultured cells. Since sm 22, as well as the smooth muscle-associated alpha-actin isoform and calponin are also expressed, IMR-90 cells must now be classified as myofibroblasts. When cultivated on a rubbery silicon surface, IMR-90 cells respond to stimulation with a rate of contraction which is considerably faster than that exhibited by fibroblasts derived from other established lines. Taken together, the regular longitudinal orientation of the adhaerens junctions, the stable expression of metavinculin, and the rapid speed of shortening in IMR-90 cells suggest, by implication, that the periodicity of actin attachment sites is a fundamental determinant of contractile efficiency in smooth muscle cells; this spacing may be mediated by metavinculin.