Radioisotopic cation transport studies are described in a family whose erythrocytes had previously been found to show an abnormal net efflux of potassium when cooled to room temperature. This net efflux effect, which was inherited as an autosomal dominant trait, was associated with a few target cells on the blood film and a mild compensated haemolytic state. Measurements of intracellular electrolyte concentrations, cell water and of Na+ and K+ transport rates across the membrane at 37 degrees C were consistent with a diagnosis of mild hereditary xerocytosis. Studies of cation transport in the temperature range 20-37 degrees C revealed that the fluxes attributable to the Na+-K+ pump showed a temperature dependence comparable with that in normal cells, but that the ouabain plus loop-diuretic insensitive fluxes of K+, which probably represent the 'passive diffusional leak' to K+, were less sensitive to temperature than normal over the range 20-37 degrees C. These findings were held to account for the net efflux effect previously reported.