A simple procedure is presented to describe accurately the whole blood CO2 dissociation curve on linear content (CCO2) and pressure (PCO2) coordinates with an exponential equation (CCO2 = K . PCO2b). A single coordinate and the hemoglobin concentration, Hb, are required. Whole blood CCO2 can be calculated from values for pH, PCO2, Hb and O2 saturation by empirically accurate equations. The mathematical description of the CO2 curve was employed to quantitate the in vivo Haldane factor (fH) from simultaneous arterial and mixed venous blood samples in 20 healthy exercising subjects. The mean +/- SE was 0.28 +/- 0.03 (vol. % delta CCO2/vol. % delta HbO2). In 20 patients with severe obstructive lung disease fH was 0.29 +/- 0.08 when calculated from arterial samples while breathing air and 100% O2. Values for fH were not related significantly to acid-base status or Hb as suggested by previous workers. By assuming these or other values for fH, the in vivo change in blood PCO2 resulting from a given change in oxygenation can be predicted.