The model of human Fc fragment was refined at 2.9-A resolution. Two different automated procedures for crystallographic refinement were used [Deisenhofer, J., & Steigemann, W. (1975) Acta Crystallogr., Sec. B B31, 238; Jack, A., & Levitt, M. (1978) Acta Crystallogr., Sect. A A34, 931]. The final R value is 0.22. The dimer of CH3 domains closely resembles the CH1-CL aggregate in Fab fragments. There is no contact between CH2 domains. The contact between CH2 and CH3 domains has about one-third of the size of the CH3-CH3 contact. The carbohydrate, a branched chain of nine hexose units, covers parts of the C-contact face of the CH2 domain, shielding hydrophobic residues on this surface. Six atoms of the carbohydrate are within hydrogen-bonding distance of atoms in the CH2 domain. Crystallographic refinement of the complex between Fc fragment and fragment B of protein A from Staphylococcus aureus reduced the R value of the model is 0.24. A major part of the structure of fragment B consists of two alpha helics; the rest of the polypeptide chain is folded irregularly. In the crystal, fragment B forms two contacts with Fc fragment molecules. Contact 1 involves residues from both helices of fragment B, and residues from the CH2 and CH3 domains of FC, and is predominantly hydrophobic. Contact 2 is smaller than contact 1. Residues from the second helix and adjacent residues of fragment B and residues only from the CH3 domain of Fc contribute to contact 2. The nature of contact 2 is mainly polar and includes a sulfate ion. There are strong arguments that contact 1 is the fragment B-Fc contact formed in solution under physiological conditions, while contact 2 is a crystal contact.