Carrier-containing membranes which can pump a specific ion from a region of low concentration into a region of high concentration have been constructed. This pumping is a large effect, proportional to the amount of carrier present. The main requirement for this type of transport is that the carrier react competitively with two simultaneously diffusing solutes. One solute is pumped; the second supplies the energy for the pumping. A theory based on the assumption that the two carrier reactions are fast predicts that the amount pumped is proportional to the carrier concentration, that the pumping stops when the concentration gradient of the energy supplying solute reaches zero, and that the effect goes through a maximum when the solubility in the membrane of the solute being pumped decreases. These predictions are in agreement with experimental results for sodium transport across membranes containing a variety of carriers, including stearic acid, lecithin, and monensin.