AbstractA method is described for the measurement of the CO content of blood, particularly at low concentrations. The error of a single determination was 1–3 per cent of the total amount of CO (range 0.7–70 μl) present in the analyzed sample. At an ordinary CO content of about 0.1 ml CO/100 ml blood the error was about ± 0.002 ml CO/100 ml blood (S.D.). Different procedures for the release of CO from the blood were compared. With K3Fe(CN)6and H2SO4equal and slightly smaller amounts of CO were obtained than with KH(JO3)2, if blood with a low CO content was analyzed. Evidently KH(JO3)2, although less than some other chemical agents (K2Cr2O7and NaOH), causes extra CO formation from the blood. In spite of the error due to this extra CO formation there were advantages to use KH(JO3)2for analysis in higher ranges of CO concentrations, when this small error became insignificant. If extraction of CO from the blood with reagents such as K3Fe(CN)6, H2SO4or KH(JO3)2took place at a high O2partial pressure slightly larger amounts of CO were obtained than with extraction in“vacuum” or in an atmosphere of N2. The results indicate that small amounts of CO may be formed during procedures used for the release of CO from blood, probably due to oxidative break down of h