In an attempt to estimate the influences of CO on the CO2 Bohr effect and the 2,3-diphosphoglycerate (DPG) effect linked to the reversible binding of O2 to the hemoglobin molecule (Hb), O2 dissociation curves of human blood in the presence of CO were investigated at 37 degrees C over a DPG concentration ranging from 2.2 to 4.3 mmol/(1RBC) and a pH range of 7.2 to 7.6. The sample with a low DPG concentration was made by incubating whole blood for 6 hrs, whereas the saturation of Hb with CO, SCO in the sample was adjusted by anaerobically mixing completely carboxygenated blood with that free of O2 and CO so as to give the final SCO at either 0, 10, 15, 20, 40 or 50%. The blood samples thus prepared were diluted at 1:100 in isotonic buffer solution and were equilibrated with gas mixtures containing O2 ranging from 1 to 9% and CO2 from 4 to 9%. The SO2 and SCO values of diluted red cell suspensions were examined by means of a dual-wavelength spectrophotometric method based on the isosbestic points for reduced Hb, HbO2 and HbCO. The extinction difference at 562.7 and 578.0 nm was monitored as a measure of SO2, while that at 570.1 and 584.6 nm was recorded for determining SCO. Fitting the experimental results by the Hill equation, the coefficients describing allosteric interaction induced by CO2 hydration and by DPG in the presence of both O2 and CO were calculated for the total saturation, ST defined as the sum of SO2 and SCO, ranging from 20 to 90%.(ABSTRACT TRUNCATED AT 250 WORDS)