Many biological materials and cell substrates are very soft (Young's modulus <500 Pa) and it is difficult to characterize their mechanical properties. Here we report local elasticity of the surface layers of Matrigel films used for cell culture. We used anew measurement technology, mechanical imaging interferometry, to obtain point mechanical measurements over micron-sized areas. The median values of 650 +/- 400 Pa (# measurements, n = 50), determined by the Hertz contact model, agree well with bulk measurements; however, on the microscale, the films were heterogeneous and contained regions distinctly stiffer than average (1-2 kPa). The first measurement of yield strengths of 170 100 Pa (n = 43) indicates that Matrigel films deform plastically at stress levels of similar scale to cell tractional forces.