The ethylene oxide adduct formed on the N-terminal valine in haemoglobin was investigated as a biological monitor of tobacco smoke intake. The modified method developed for the determination of the hydroxyethylvaline adduct (HOEtVal) involved reaction of globin with pentafluorophenyl isothiocyanate, extraction of the HOEtVal thiohydantoin product, derivatization of this by trimethylsilylation and quantitation by capillary gas chromatography with selective ion monitoring mass spectrometry using a tetradeuterated internal standard. The method was applied to globin samples from 26 habitual cigarette smokers and 24 non-smokers. There was a significant correlation between cigarette smoke intake as measured by the average number of cigarettes smoked per day and HOEtVal levels (r = 0.537, p less than 0.01). Background levels were found in non-smokers (mean 49.9 pmol/g Hb, range 22-106 pmol/g Hb). Smoking increased these levels by 71 pmol/g Hb/10 cigarettes per day. Cotinine levels in plasma of the smokers were determined by GC-NPD using 2-methyl-4-nitroaniline as internal standard. For non-smokers cotinine was determined by GC-MS selective ion monitoring using d3-methylcotinine as internal standard. There was no correlation between number of cigarettes smoked per day and cotinine levels (r = 0.297, p greater than 0.05) although cotinine was correlated with HOEtVal (r = 0.43, p less than 0.01). The HOEtVal adduct levels thus appear to be a suitable biomonitor for exposure to hydroxyethylating agents in cigarette smoke, reflecting an integrated dose over the erythrocyte lifetime. This is in contrast to plasma cotinine determinations which reflect only the previous day's exposure to nicotine in smoke.