Gas chromatography was used to quantitate CO in air and also as an indirect means of determining %COHb in blood. The blood was then used to calibrate four CO-Oximeters used in a survey to determine average COHb levels in various segments of the U.S. population. Mean differences, both between the two methods of analysis and between pairs of CO-Oximeters were less than 0.1% COHb saturation. COHb values obtained on consecutive days using one CO-Oximeter were repeatable within a S.D. +/- 0.13% COHb.