Trypsins were crystallized by using membrane crystallization techniques, both in static and dynamic configurations. Crystal size distribution analysis revealed the high uniformity in size of the products that is achievable. Crystals produced by using membrane crystallization methods are more uniform than those obtained in conventional batch experiments. Crystals produced in forced solution flow are even more uniform than those obtained in a quiescent membrane system. Typical values obtained of percent standard deviation and Span are comparable to or even better than those reported in the literature. Crystal growth rate is favorably affected by solution convective motions in forced solution flow, by increasing solution velocity up to a maximum, which, in the best case, has been observed to be ca. 1500 mu m/s. These high values of suitable velocities are characteristics of the membrane system; as with traditional method, growth rate deceleration was already visible at 250 mu m/s. The maximum seems to be shifted according to the kinematic viscosity of the crystallizing solutions. Some crystals were assessed by X-ray crystallographic analysis, which demonstrated that dynamically grown crystals show no loss in diffraction quality with respect to static membrane crystallized proteins.