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 a new 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.