To obtain a general understanding of the effect of intermolecular interactions on the mechanisms of two-dimensional protein crystallization, we grow protein crystals and elicit a bulk molecular manipulation by changing system pH. Two-dimensional crystals of the bacterial protein streptavidin grown on a biotinylated lipid monolayer at an air-water interface, in the presence of the noncrystallizable impurity avidin, exhibit crystallographic and morphological changes as a function of subphase pH. Large two-dimensional crystalline arrays form within minutes across a pH range from 1.5 to 11. Crystals exhibit different pH-dependent structures, lattices with P1 symmetry for 1.5 < pH < 5, P1 and P2 lattices for 5 < pH < 6, and C222 lattices for 7 < pH < 11. P1 crystals nucleate rapidly and form thin needle-shaped crystals consistent with a strong growth anisotropy between the two crystallographic growth directions. C222 crystals grow more isotropically and exhibit H- and X-shapes. The nucleation rates and aspect ratios of C222 crystals are also pH-dependent, both properties increasing with increasing pH. The transition from C222 to P1 or P2 crystals can be accomplished in minutes by lowering the system pH. The reverse transition, however, does not occur subsequent to a corresponding increase in system pH. Instead, new C222 crystals form, but no reconfiguration of existing crystals is observed.