Several culture models exist to study fibroblasts' ability of to reorganize and contract collagen matrices in vitro. In the two-step attached contraction model, a polymerized collagen matrix containing cells remains attached to the culture dish during contraction. Mechanical tension develops during contraction, and cellular stress fibers assemble. The model combines an initial period of attached matrix contraction leading to mechanical loading, followed by release of the matrices, resulting in mechanical unloading and further contraction as mechanical stress dissipates. Cell Biolabs’ Collagen-based Contraction Assay Kit provides a simple system to assess cell contractivity in vitro and screen cell contraction mediators using the two-step attached model. Each kit provides sufficient quantities to perform up to 24 assays in a 24-well plate. The kit can also be used for culturing cells in a 3D collagen matrix.