The av. content of CO in the blood of 18 persons living in New York City under conditions of minimal exposure was found to be 0.27 vol. %. This represents about 1-1.5% of Hb combined with CO. The av. content of CO in the blood of 12 persons confined to a state institution in an ideal rural locality was found to be 0.24 vol. %. Most of these showed a Hb saturation of less than 1%. The av. content of CO in the blood of 12 New York City street cleaners was found to be 0.69 vol. %. This represents about 3% Hb saturation. Two taxi-cab drivers were found to have on several occasions a CO content ranging from 1.47 to 4.33 vol. %., a Hb saturation of 8-19%. Tobacco smoking appreciably increases the CO in the blood and cannot be ignored in the interpretation of laboratory results.