1. The Na+ and Cl− transport systems of human erythrocytes have been compared for their sensitivities to diuretics known to act in the ascending limb of Henle's loop. In addition, chemical analogues of ‘loop’ compounds and also diuretics which act in other areas of the nephron have been examined.
2. The Na+ transport system lacks specificity with respect to inhibition by ‘loop’ diuretics and also a related chemical analogue studied at equivalent concentrations.
3. The Cl− transport system is inhibited, at low concentrations, by diuretics known to act in the ascending limb of Henle's loop.
4. Erythrocyte Cl− transport offers a useful model with which to study the biochemical action of diuretics.