Adhesive interactions between cells and collagen required for contraction are mediated by a2/31 integrins (Schiro et al., 1991; Klein et al., 1991). Tensile force was found to depend on an intact actin cytoskeleton (Bell et al., 1979; Bellows et al., 1982; Guidry and Griunell, 1985) and myosin light chain kinase (MLCK) activity (Ehrlich and Griswold, 1984; Van Bockxmeer et al., 1984; Ehrlich et al., 1991). Collagen matrix contraction requires serum (Steinberg et al., 1980; Guidry and Grinnell, 1985), which indicates that cell contractility can be regulated by extracellular factors. The activity of serum can be replaced or enhanced by purified growth factors. For instance, transforming growth factor (TGF-/~) stimulates contraction of both floating and anchored collagen matrices (Montesano and Orci, 1988; Finesmith et al., 1990; Fukamizu and Grinnell, 1990). TGF-/~ also has been reported to promote fibroblast differentiation into myofibroblasts (Ronnov-Jessen and Petersen, 1993; Desmouli et al., 1993). Platelet derived growth factor (PDGF) stimulates matrix contraction (Clark et al., 1989; Gullberg et al., 1990), but by a mechanism independent of TGF-/3 (Tingstrom et al., 1992). Factors that inhibit matrix contraction include fibroblast growth factor (Finesmith et al., 1990; Dubertret et al., 1991) and interferon 3'(Gillery et al., 1992). In general, the downstream signaling mechanisms by which growth factors regulate matrix contraction are unknown, but protein kinase C probably is involved (Danowski and Harris, 1988; Guidry, 1992),