1. Human leucocytes were isolated from venous blood and resuspended in the subject's plasma.
2. Intracellular pH measurements were made in vitro by the dimethyloxazolidinedione technique in 13 healthy subjects and in 11 subjects with renal failure and metabolic acidosis.
3. The intracellular pH of the healthy subjects was found to be 7·07 (sd 0·04), significantly lower than that of the patients, which was 7·11 (sd 0·03). For all estimations plasma Pco2 was maintained at approximately 5·5 kPa. The methodological and possible metabolic reasons for this difference in intracellular pH are discussed.