X-ray analysis at 3.2-A resolution revealed that the Mcg IgG1 (lambda chain) immunoglobulin is a compact T-shaped molecule. Because of the hinge deletion, the Fc fragment lobe is pulled tightly upward into the junction of the Fab arms. Along the molecular twofold axis, the Fab arms are joined by an interchain disulfide bond between the two light chains. The antigen combining sites consist of large irregular cavities at the tips of the Fab regions. Potential complement (C1q) binding sites on Fc are sterically shielded by the Fab arms, but putative attachment sites are accessible for docking with the FcRI receptor on human monocytes and with protein A of Staphylococcus aureus.