The incorporation of impurities during protein crystallization is one of the main obstacles that prevents the growth of high quality crystals. Mass transport has been shown to affect the incorporation of impurities. Here we used a special growth configuration that enables the simultaneous investigation of the two main means of mass transport, diffusion and convection, under otherwise identical conditions. Two polymorphic forms of hen egg-white lysozyme were crystallized using this configuration in the presence of different types of impurities at various degrees of contamination. We found that even in the presence of impurities that are not easily segregated, the diffraction quality of crystals grown under diffusion-limited conditions is better than that of those grown in the presence of natural convection. The results also reveal a significant difference in impurity uptake for the different polymorphic forms of the same protein. The combined results show that also in the presence of large fractions of impurities, the more perfect crystals grow when the rate of accretion of molecules is slow and orderly, as accomplished under diffusion-limited conditions.