1. Frusemide-sensitive sodium and potassium transport by normal human leucocytes has been studied in vitro by both isotopic and net flux techniques.
2. In physiological media the leucocyte exhibits a frusemide-sensitive influx of sodium and potassium of equal magnitude compatible with a 1:1 co-transport system.
3. Cells exposed to zero external sodium and potassium (osmolality maintained with choline) demonstrated a frusemide-sensitive sodium and potassium efflux.
4. Frusemide-sensitive potassium influx was dependent on the presence of external sodium but frusemide-sensitive sodium influx persisted unchanged in the absence of external potassium.
5. Frusemide-sensitive potassium influx was dependent on external chloride but frusemide-sensitive sodium influx was chloride-independent.
6. These last two observations make it likely that the frusemide-sensitive pathway is capable of operating in modes other than sodium-potassium co-transport.