The osmotic second virial coefficient, B 22 , has become the quantity most widely used in developing a rational understanding of protein crystallization. In this work a novel method of measuring B 22 using self-interaction chromatography (SIC) is presented that is at least an order of magnitude more efficient than traditional characterization methods, such as static light scattering. It is shown that SIC measurements of second virial coefficients for BSA are in quantitative agreement with static light scattering results. The measured virial coefficient for both BSA and myoglobin reveal a surprisingly narrow range of concentrations of ammonium sulfate that promote weakly attractive interactions that are optimal for crystallization. Using the virial coefficient information, myoglobin crystals were obtained by ultracentrifugal crystallization in a rational and rapid manner.