A new modified method for the spectrophotometric determination of carboxyhemoglobin was devised. Normal blood samples from 50 subjects including 20 no smokers were analyzed, and the coefficinent C which is the mean ratio of absorbance of carboxyhemoglobin to oxyhemoglobin at 419 mμ was experi-mentally determined. The following equation for the calculating of the concentration of carboxyhemoglobin was derived from the application of the coefficient. X (%)=131×(1.76×Ex/E₁₀₀-1) In this equation, Ex is the absorbance at 419 mμ of a blood sample diluted to 2000 times, and E₁₀₀ is the one of the same diluted blood saturated with carbonmonoxide. The technique of saturating carboxyhemoglobin was improved in the method. Dilution of blood sample with carbonmonoxide water showed better results in the point of saturation of carboxyhemoglobin and its deviation than the usual method blowing carbonmonoxide gas into diluted blood. It was also demonstrated that the change of the absorption spectrum and the decrease of the maximum absorbance of carboxyhemoglobin are caused by blowing enormous volume of gases such as oxygen, nitrogen and carbondioxide.