A new technique, which allows to characterize the local strain field over a domain representative of the microstructure of a heterogeneous material, is described. It is based on scanning electron microscopy, microelectrolithography, image analysis and in situ tensile tests. The in-plane components of the local strain field are characterized by their averages per phase and their distribution functions. The results are accurate for global strains between 5 and 15%. It is also possible to get contour plots of these components of the local strain field over the considered domain. The obtained strain maps give a powerful qualitative information on the strain localization modes during the deformation. This technique has basically been developed for two-phase elastoplastic materials, namely iron/silver and iron/copper blends, submitted to uniaxial tensile tests; it could also be used for polycrystals or other composite materials and for other mechanical tests.