Rat liver microsomal fractions have been equilibrated in various types of linear density gradients. 15 fractions were collected and assayed for 27 constituents. As a result of this analysis microsomal constituents have been classified, in the order of increasing median density, into four groups labeled a, b, c, and d. Group a includes: monoamine oxidase, galactosyltransferase, 5'-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase, and cholesterol; group b: NADH cytochrome c reductase, NADPH cytochrome c reductase, aminopyrine demethylase, cytochrome b(5), and cytochrome P 450; group c: glucose 6-phosphatase, nucleoside diphosphatase, esterase, beta-glucuronidase, and glucuronyltransferase; group d: RNA, membrane-bound ribosomes, and some enzymes probably adsorbed on ribosomes: fumarase, aldolase, and glutamine synthetase. Analysis of the microsomal fraction by differential centrifugation in density gradient has further dissociated group a into constituents which sediment more slowly (monoamine oxidase and galactosyltransferase) than those of groups b and c, and 5'-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase, and the bulk of cholesterol which sediment more rapidly (group a2). The microsomal monoamine oxidase is attributed, at least partially, to detached fragments of external mitochondrial membrane. Galactosyltransferase belongs to the Golgi complex. Group a2 constituents are related to plasma membranes. Constituents of groups b and c and RNA belong to microsomal vesicles derived from the endoplasmic reticulum. These latter exhibit a noticeable biochemical heterogeneity and represent at the most 80% of microsomal protein, the rest being accounted for by particles bearing the constituents of groups a and some contaminating mitochondria, lysosomes, and peroxisomes. Attention is called to the operational meaning of microsomal subfractions and to their cytological complexity.