A national survey was conducted in 1969-1972 for the purpose of determining the range of carboxyhemoglobin (COHb) in various segments of the American Population. Venous blood samples were obtained from 29,000 adults at arbitrarily chosen blood bank collection sites. These blood samples were then analyzed for COHb content. From every tenth participant, an alveolar breath sample was obtained for CO analysis and correlation with the COHb. At the time of blood collection, detailed questionnaires were completed so that the influence of the following variables could be assessed: age, weight, height, sex, race, health status, smoking habits, place of residence, occupation, place of work, meteorological conditions, background CO concentration at sampling site, and location and time of sampling. For comparative purposes, COHb measurements were also made on samples from 1,525 blood donors in 13 smaller communities in Vermont and New Hampshire, and on samples obtained from volunteers breathing CO-free air and air with known concentrations of CO.