Mesenchymal-like cells in the stroma of breast cancer may arise as a consequence of plasticity within the epithelial compartment, also referred to as epithelial-mesenchymal transition, or by recruitment of genuine mesenchymal cells from the peritumoral stroma. Cells of both the epithelial compartment and the stromal compartment express alpha smooth muscle actin (alpha-sm actin) as part of a myoepithelial or a myofibroblastic differentiation program, respectively. Moreover, because both epithelial- and mesenchymal-derived cells are nontumorigenic, other means of discrimination are warranted. Here, we describe the contraction of hydrated collagen gels as a rapid functional assay for the distinction between epithelial- and mesenchymal-derived stromal-like cells irrespective of the status of alpha-sm actin expression. Three epithelial-derived cell lines and three genuine mesenchymal-derived breast cell lines were plated on top of hydrated collagen lattices. Reduction in gel height was measured every hour for 6 h and after 22 h using an x-y-z automated position table. Significantly, the epithelial-derived cells, irrespective of a high alpha-sm actin expression, had a fivefold lower contractility (10.0% reduction in gel height) than their true mesenchymal counterparts (53.1% reduction in gel height). To test whether at all force generation could be induced in the nonmesenchymal cells by alpha-sm actin, transductions were performed to obtain a tetracycline-dependent expression. Expression under these conditions did not augment contractility. It is concluded that epithelial-derived mesenchymal-like cells are functionally defective within a connective tissue environment irrespective of an apparent contractile phenotype.