Peak-to-peak distances between two dense lines of the unit membranes of cell organelles were measured on electron micrographs. These distances were compared with corresponding measurements on the plasma membrane and assigned a percentage value. The comparison between organelle and plasma membrane was always carried out with the same negative, in order to exclude as far as possible errors due to differences in focus or other causes. It was revealed by this study that the membranous structures of the cell can be classified into two groups, one thicker and one thinner. Unit membranes of the thicker group (synaptic vesicles, vesicles and capsules of multivesicular bodies, Golgi vesicles) were not significantly different in thickness from the plasma membrane. Unit membranes of the thinner group (mitochondria, nuclear membranes, Golgi lamellae, endoplasmic reticulum), however, were between 85 and 90 per cent of the thickness of the plasma membrane.