DANIEL S. FRIEND Department of Pathology, University of California School of Medicine, San Francisco, California 94143

In their classical experiments on membrane fluidity, Frye and Edidin (1) triggered further research on the fluid mosaic model of membrane structure (2) in numerous other laboratories. Since then, investigators in this field have also emphasized the planar, functionally specialized mosaics within the generally fluid bilayer of the membrane. These mosaics comprise areas where specific groups oflipids, proteins, glycoconjugates, and sterols aggregate to mediate specific functions. To cite several different facets of the membrane-domain concept, we can recall the now familiar work of Goldstein et al.(3) on the distinctive participation of coated pits in receptor-mediated endocytosis. Other thought-provoking membrane regionalities are the rosette-particle arrays instrumental in mucocyst secretion in Tetrahymena (4), the need for specific lipids for the complete activity of cytochrome c oxidase and other membrane-related enzymes (5), and the variations of phospholipid content in continuous membrane systems.(Consider the nuclear envelope and the rough-surfaced endoplasmic reticulum [6l.)