Growth of two-dimensional streptavidin crystals at the air-water interface has been used to study protein molecular interactions at various pH values. Between pH 5 and 6, several unique crystal domain shapes are visible with fluorescence microscopy. Characterization of a chiral domain with transmission electron microscopy reveals a composition of two coexisting crystal types. The bulk crystal, one previously seen at pH 4 (space group P1) (Hemming ct al. J. Mol. Biol. 1995, 246, 308), has interspersed within it at approximately 5% aerial extent a new crystal type with unit cell parameters a = 116 Angstrom, b = 58 Angstrom, and y = 107 degrees (space group P2). This new form exists in the chiral domains as long narrow crystals and is characterized by relatively weak and anisotropic molecular interactions. The packing arrangement for the new P2 crystal at pH 5-6 exhibits characteristics of the pH 4 crystal and of the C222 crystal (Darst ct al. Biophys. J. 1991, 59, 387) obtained at pH 7.