Adhesive cells including fibroblasts produce contractile forces to the underlying substrate for their locomotion. Such forces are not only high enough to deform compliant membranes such as silicone but also bend rigid micro-cantilevers. The cell-induced bending of silicon micro-cantilevers was determined using laser deflection during trypsin treatment. The observed cantilever relaxation corresponds to a contractile cell force of (16±7) µN per rat-2 fibroblast. The cantilever bending approach represents a unique method for the determination of contractile cell forces on any kind of rigid substrate in desired (physiological) environment. Hence, the fundamental technique allows building cell-based biosensors or should provide a quantitative parameter for characterising the cyto-compatibility of load bearing implant surfaces.