Abstract Membrane fragments were isolated from a homogenate of Phaseolus vulgaris cotyledon tissue by a centrifugation scheme designed to yield purified plasma membrane. The membrane fraction was enriched in basal ATPase (EC 220.127.116.11) activity by 3–6-fold relative to homogenate on a specific activity basis and in (Na +-K +)-stimulated ATPase activity by 6–9-fold. Glucose-6-phosphatase (EC 18.104.22.168) and 5′-nucleotidase (EC 22.214.171.124), markers for microsomal membranes in this tissue, as well as the mitochondrial enzyme succinate dehydrogenase (EC 126.96.36.199) were relatively absent. By electron microscopy it was apparent that the fraction was a clean preparation of membranes with primarily vesicular conformations. The increased specific activity of the basal enzyme and in particular that of the (Na +-K +)-stimulated ATPase, together with the relative absence of microsomal and mitochondrial membranes, suggest that the preparation is partially purified plasma membrane.