1. In sixteen patients with severe chronic renal failure the rate constant for total sodium efflux from leucocytes was significantly reduced compared with that in thirty control subjects. This difference lay chiefly in the glycoside-sensitive (‘active’) moiety of sodium efflux.
2. In sixteen patients receiving regular haemodialysis, the rate constant for total sodium efflux from the leucocyte was significantly greater than in the undialysed uraemic patients though still subnormal.
3. In individual patients, an increase in sodium efflux could be detected as early as 1 week after regular haemodialysis was started.
4. These results are compatible with the existence of a dialysable molecule in uraemic plasma affecting leucocyte sodium transport.