The recent studies have clearly established two types of active transport systems. One type is membrane-bound and can be observed in membrane vesicles and the other type is osmotic-shock-sensitive and requires binding proteins to produce active transport. It appears that the membrane-bound systems derive cellular energy from the energy-rich membrane state which can be formed from respiration or ATP-hydrolysis, while the binding protein systems are more directly coupled to phosphate bond energy derived from glycolysis or oxidative phosphorylation. The following conclusions concerning the role of the binding proteins are offered: 1. The binding proteins are present in relatively large amounts (approximately 10(-6) or 10%-5) M) and appear to reside in the periplasmic space. 2. They do not appear to be involved in solute translocation steps, although they cantain a second binding site that could interact with membrane components. 3. The binding proteins appear to increase the affinity of the transport system for the solute by interacting with a membrane component. This may substrate for the membrane transport system.