The atomic force microscope (AFM) was used to measure the surface free energy of streptavidin crystals. To this purpose, AFM force-distance curves were acquired employing a streptavidin-functionalized tip. The jump-off-contact minimum in the force-distance curves yielded an adhesion force of 240 pN between the tip and the protein crystal. The adhesion force is related to the work of adhesion, which, in the case of identical surfaces, is just twice the surface free energy. The Bradley and Johnson-Kendall-Roberts theories of adhesion mechanics predict two different relations between the pull-off force and the adhesion work for rigid and elastic bodies. On the basis of recent computational studies of adhesive contact between deformable bodies, we show that the Bradley equation holds in our experimental conditions. We also evaluate the number of molecules interacting during the pull-off, considering the sample area on which the stress is applied. From the force measurement we obtain a surface free energy of 0.16 +/- 0.04 mJ/m(2), in good agreement with the value obtained from analysis of the crystal growth morphology.