The in vivo and in vitro oxygen-binding capacity of haemoglobin was determined on 10 occasions in nine patients who required mechanical ventilation. The in vitro sample was tonometered with 97% oxygen for 10 min and then with air, while the in vivo sample was obtained after 20 min of lung ventilation with pure oxygen. Subsequent laboratory procedures were identical for both samples. The mean oxygen-binding capacity of haemoglobin in vitro and in vivo samples were almost equal (1.365 +2- 0.010 and 1,366 +/- 0.007 ml per g Hb). When the measured inactive fractions of haemoglobin (carboxy- and methaemoglobin) were taken into account, these values increased to 1.392 +/- 0.005 and 1.392 +/- 0.007 ml per g Hb respectively.