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ABSTRACT1. In a membrane transport system containing a mobile carrier with affinities for two substrates a concentration gradient with respect to one of the substrates under certain conditions is able to induce an "uphill" transport (against the concentration gradient) of the other.2. In a kinetic treatment quantitative conditions for such a "flow-induced uphill transport" and some of its characteristics are derived.3. Experimentally the uphill transport of labelled glucose induced by a concentration gradient for rnannose or unlabelled glucose is demonstrated in the human red cell.4. It is shown that the flow-induced uphill transport is a feature characteristic for mobile carrier systems only and is not to be expected in systems in which the substrate is bound to a fixed membrane component ("adsorption membrane"), although such a system may yield identical transport kinetics. Also with respect to Ussing's flux ratio the two systems are different, the adsorption membrane meeting Ussing's criterion, the carrier membrane not.5. It is concluded that the transport system in the human red cells must contain a mobile carrier, identical for glucose and mannose.Through the epithelial cells of kidney (1) and intestine (2) sugars may be transported "actively" in the thermodynamic sense; i.e., against the concentration gradients. Rosenberg (3) has discussed transports of this kind in terms of thermodynamic quantities and potentials. He showed that the mere fact of a transport occurring against the concentration gradient (hereafter termed "uphill") allows the conclusion that a "carrier" in a broader sense is involved in the transport mechanism. In addition, in the case of sugar absorption, other indirect criteria (4) likewise point to carrier mechanisms (limited capacity of the transport, competition, kinetics, effect of enzyme inhibitors).In the erythrocyte, the transport of sugars under conventional experimental * This work was carried out with partial support from the Lalor Fellowship Foundation which is gratefully acknowledged. 289]'.

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