We have developed a simple method for rapid determination of carboxyhemoglobin concentration in blood, by use of double-wavelength spectrophotometry. The absorbance of both oxyhemoglobin and reduced hemoglobin are instrumentally nulled out in this procedure, so that the absorption signal reflects HbCO and only a single spectrophotometric reading is required. This procedure is accurate in the range encountered in ambulatory persons as well as in the "toxic" range. Results correlated well with the dithionite reduction procedure. The stability of carboxyhemoglobin in solution was investigated and concentrations of carbon monoxide in groups of smokers and nonsmokers were determined.