1. A triple-isotope method for measuring initial 22Na uptake rates in unactivated human leucocytes in the presence of serum, using 3H2O, [14C]sucrose and 22Na, is described.

2. This 22Na influx is inhibited by amiloride, with half-maximal inhibition at a concentration of 10−5 mol/l.

3. About 70% of the 22Na influx in normal human leucocytes is inhibited by 1 mmol/l amiloride, with an external Na+ concentration of 10 mmol/l.

4. External Na+ antagonizes this inhibition by amiloride. The Km for external Na+ is 9 mmol/l.

5. Intracellular acidification from either external Na+ depletion or ammonium chloride incubation leads to an activation of amiloride-sensitive Na+ influx.

6. The amiloride-sensitive Na+–H+ antiport provides the major influx pathway for Na+ in unactivated human leucocytes in the presence of serum.

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