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Of the many techniques and methods available for measuring carbon monoxide in blood, one of the easiest involves the use of the IL-182 CO-Oximeter, a specialized triple-wavelength spectrophotometer. Although easy to use, the calibration procedure can be lengthy and impractical if performed routinely to ensure accuracy. A method is described for an equivalent calibration by using a programmable calculator or minicomputer. The new method allows rapid and easy calibration as well as the analysis of unknown blood samples. Three anticoagulated blood samples, each containing as little as 2 ml, are required to calibrate. One sample is chemically reduced, one is tonometered in pure oxygen, and the third is tonometered in pure carbon monoxide. The optical densities of these three known calibration samples, determined at each of the three specific wavelengths used by the IL-182, allow one to derive the constants used in the equations needed to solve for an unknown sample.