Non-contractile cells are able to exert 'tensional' forces on collagen substrate. Although such forces have been implicated in contraction during tissue repair their importance and mechanism of action are difficult to assess without quantitative data. Currently, the most widely used model is not a direct measure of force and has serious deficiencies as a model of wound contraction. A Culture Force Monitor, CFM, has been developed in this study, to measure directly the forces generated by fibroblasts and other cell types in culture. Under model conditions, a peak force of 1 x 10(-10) Newton/cell was generated (assuming participation of all cells) over a range of cell densities. Contraction of a collagen gel over 2 days was in 3 phases; initial contraction, linear increase and equilibrium. The final, maximum force was produced at the equilibrium phase (24 h) and balanced the restraining force of the CFM. Cell force responses during the initial and linear phases (corresponding to cell attachment) indicated that contraction of the cells responded in a rapid and subtle manner to changes in the mechanical properties of their substrate.