Carboxyhaemoglobin (COHb) levels in tobacco smokers vary throughout the day since they are affected by the pattern of tobacco consumption and the rate at which COHb is eliminated. A method is described whereby a single COHb measurement together with a recent smoking history may be used to estimate the average COHb "boost" produced by each cigarette, the total daily carbon monoxide (CO) uptake from smoking, and the mean COHb level throughout the day. These three indices of tobacco smoke absorption were estimated in nine healthy cigarette smokers on different days, each set of three estimations being derived from separate COHb determinations. The indices were reasonably reproducible within the same person, and the differences between people were statistically highly significant (P less than 0-001). For example, the estimates of mean daily COHb level resulting from smoking ranged from 0-7% to 9-3% in smokers who smoked 15 to 40 cigarettes a day. These differences are sufficiently large to distinguish possible differences in the risk of developing diseases such as ischaemic heart disease which may result from the inhalation and absorption of tobacco smoke. The suggested indices also depend less on the time of the blood test and on the daily pattern of smoking than a COHb level alone. The ratio of the COHb boost to the CO yield of a cigarrette may reflect depth of inhalation more accurately than a smoker's self-assessment. Moreover there was little correlation between these two measures of inhalation in the nine subjects studied.