Receptors at the surface of bacteria are involved mainly in the interaction of the cell with macromolecular entities. They are essential for the infection by bacterial viruses, for the killing by toxic proteins, for conjugation and transformation, for binding of bacteria to tissues, and for the defense of organisms against bacterial infections by antibodies, complement, or phagocytosis. The first powerful method employed to obtain an insight into the structure of the bacterial cell envelope was electron microscopy of shadowed preparations and of stained ultrathin sections of cells and of isolated envelopes. Considerable progress in the elucidation of the molecular organization and the assembly of the outer membrane was achieved when methods were devised to isolate the outer membrane components free of cytoplasmic membrane components. Gram-negative bacteria are surrounded by two boundary layers: the cytoplasmic membrane and the outer membrane.