Our understanding of the molecular organization of even the simplest mammalian cell membrane--that of the red blood cell--is still incomplete, yet we are reasonably sure of where and how the major components are arranged. The so-called integral membrane proteins are the most likely candidates to serve as receptors and transport sites of surface membranes, and these may be organized as macromolecular complexes with parts of their polypeptides in contact with both sides of the bilayer. These complexes may be either mobile or fixed, and if the latter, perhaps by specific interactions with other membrane proteins. The exposed segments of such glycoproteins could have recognition sites that confer cell, tissue, and/or species specificity created by oligosaccharides attached to the peptide backbone, or by specific amino acid sequences, or a combination of both.