Intact and homogenized yeast cells were studied in thin sections in the electron microscope to determine the fate of the plasma membrane during fractionation. Intact cells possess a unit-membrane plasma membrane closely appressed to the cell wall. After even slight physical damage following limited homogenization in dilute buffer, the plasma membrane collapses away from the wall while the intra-cytoplasmic membranes (ER, vacuolar, nuclear, mitochondrial) dilate and vesiculate. With prolonged homogenization, the plasma membrane fragments and vesiculates and becomes indistinguishable from the remains of the other membranes. Washed wall fractions consist of wall fragments with entrapped vesicles derived from all cellular membranes. Exhaustive digestion of the wall fraction with snail gut enzymes liberates some of the trapped vesicles and results in an undigested, non-membranous, inner layer of wall partially contaminated with outer wall material. Polyacrylamide gel electrophoresis indicates that proteins of the wall "membranes" are partially of mitochondrial origin.