Cellular membranes were prepared from the non-extending part of dark grown hypocotyls of Phaseolus aureus. The relative effectiveness of continuous and discontinuous sucrose gradient centrifugation for the separation of membranes was investigated. Characteristic densities of membranes were determined by the localization of enzyme activities on continuous sucrose gradients: NADH-cytochrome c-reductase for endoplasmic reticulum, beta-1-3-glucan synthetase for plasma-membrane and IDPase for dictyosomes. The difficulties involved in the application of ATPase and IDPase as specific membrane markers are discussed. Negative staining of isolated fractions indicated that intact dictyosomes could be prepared from this tissue without the use of chemical fixatives in the homogenization medium. Extraction of isolated membranes showed that carbohydrate-binding proteins (lectins) were present both in an easily removable and in a more strongly bound form. In vivo incorporation of D-[U-14C]glucose and subsequent isolation and solubilization of the different membranes showed that sugar-containing polymers could be released without hydrolytic techniques and were present in the equivalent extracts that exhibited lectin activity. The possibility of lectin-polysaccharide complexes in endoplasmic reticulum and dictyosomes and their involvement in the synthesis and transport of secretory substances by the membranes is discussed.