1. Viable human leucocytes were isolated from venous blood and suspended in artificial media. Intracellular pH measurements were made by the dimethyloxazolidinedione technique in conditions simulating ‘respiratory’ or ‘metabolic’ acid-base disturbances.
2. Normal intracellular pH was 7·11 ± 0·02 (mean ± 2 sd) at an extracellular Pco2 of 5·8 kPa and a bicarbonate concentration of 25 mmol/l.
3. ‘Respiratory’ and ‘metabolic’ acidosis caused little change in pH1 although increases in Pco2 led to relatively greater falls in pH1 than did reduction in external bicarbonate concentration.
4. ‘Respiratory’ and ‘metabolic’ alkalosis caused similar and relatively greater increases in the pH1 when compared with the response to an external acidosis.