1. In a preparation of human leucocytes maintained in tissue culture fluid, increasing the extracellular zinc concentration leads to a significant increase in both ouabain-sensitive sodium efflux and in sodium influx.
2. Cell water and sodium content do not alter significantly with increasing extracellular zinc concentration.
3. A small increase in the ouabain-insensitive sodium efflux can be demonstrated when the external zinc concentration is raised from 0·75 μmol/l to 90 μmol/l.