The use of bacterial membrane vesicles as an experimental system for the study of active transport has been discussed. Vesicles are prepared from osmotically sensitized bacteria, and consist of osmotically intact, membranebound sacs without internal structure. They retain litle or no cytoplasm. Under appropriate conditions, these vesicles catalyze the transport of a variety of solutes at rates which are comparable, in many cases, to those of intact cells. Two general types of transport systems have been elucidated in the vesicle system: (i) group translocation systems which catalyze vectorial covalent reactions; and (ii) respirationlinked transport systems that catalyze the active transport of a whole range of metabolites against an electrochemical or osmotic gradient. In E. coli membrane vesicles, the respiration-linked transport systems are coupled primarily to the oxidation of (D)-lactate to pyruvate, catalyzed by a flavin-linked, membrane-bound (D)-lactate dehydrogenase which has been purified to homogeneity. Electrons derived from (D)-lactate or certain artificial electron donors are transferred to oxygen by means of a membrane-bound respiratory chain, and respiration is coupled to active transport within a segment of the respiratory chain between the primary dehydrogenase and cytochrome. b(l). The great majority of the individual membrane vesicles in the population catalyze active transport, and the generation or hydtolysis of ATP is not involved. Under anaerobic conditions, fumarate or nitrate can be utilized in place of oxygen as terminal electron acceptors. With the exception that (D)-lactate is not always the most effective electron donor for active transport, vesicles prepared from a number of other organisms catalyze transport in a similar manner. Fluorescent dansylgalactosides are useful molecular probes of active transport in the vesicle system. These compounds are competitive inhibitors of beta-galactoside transport, but are not transported themselves. Fluorescence studies indicate that the lac carrier protein constitutes approximately 3 to 6 percent of the total membrane protein, and that it is not accessible to the external medium unless the membrane is "energized." Thus, energy is coupled to one of the initial steps in the transport process. Studies with a photoaffinity-labeled galactoside provide independent support for this conclusion. When membrane vesicles prepared from a (D)-lactate dehydrogenase mutant of E. coli are treated with (D)-lactate dehydrogenase, the enzyme binds to the vesicles and they regain the capacity to catalyze (D)-lactate oxidation and (D)-lactate-dependent active transport. The maximal specific transport activity obtained in the reconstituted system is similar in magnitude to that of wildtype vesicles. Titration studies with dansylgalactoside demonstrate that there is at least a seven- to eightfold excess of lac carrier protein relative to (D)-lactate dehydrogenase. Evidence is presented indicating that the enzyme is bound to the inner surface of native membrane vesicles and to the outer surface of reconstituted vesicles, and that the flavin coenzyme moiety is critically involved in binding. Possible mechanisms of respirationlinked active transport are discussed.