Evidence which suggests the existence of lipid regions discrete from those of the protein in mitochondrial and red blood cell membranes.

The action of phospholipase C(EC 3.1.4.3) on erythrocyte ghosts and submitochondrial particles results in a marked change in the molecular structure of the phospholipid regions of the membrane but in no change in the environments (i.e., structure) of the sulfydryl and tyrosyl residues of the membrane proteins as determined by electron spin resonance "spin label" studies. These results hold true whether the membrane is one containing a relatively high proportion of cholesterol (erythrocyte) or one containing very little cholesterol (mitochondrial). The relevance of these findings to current membrane models is discussed.