We present a new type of microinstrument allowing manipulation and mechanical perturbation of individual cells under an optical microscope. These instruments, which we call microplates, are pulled from rectangular glass bars. They have flat tips, typically 2 microm thick x 20 microm wide, whose specific shape and stiffness can be adjusted through the pulling protocol. After appropriate chemical treatment, microplates can support cell adhesion and/or spreading. Rigid microplates are used to hold cells, whereas more flexible ones serve as stress sensors, i.e. their deflexion is used to probe forces in the range of 1-1000 nN. The main advantages of microplates are their simple geometry and surface properties, and their ability to provide mechanical measurements. In this methodological paper, we give details about microplate preparation and adhesiveness, manipulation set-up, force calibration, and image analysis. Several manipulations have already been carried out on fibroblasts, including uniaxial deformation, micropipet aspiration of adherent cells, and cell-substrate separation. Our results to date provide new insights into the morphology, mechanical properties, and adhesive resistance of cells. Many future applications can be envisaged.