It is known that human hemoglobin contains low levels of N-terminal N-(2-hydroxyethyl)valine. Possible sources of this modified amino acid are exposure to ethylene oxide or other 2-hydroxy-ethylating agents. Although such processes are likely to occur endogenously, the exogenous contribution to the adduct formation is unclear. In order to explore the latter, we have analyzed N-(2-hydroxyethyl)valine in the globin of 49 pregnant women and evaluated the effect of smoking status, area of residence, and glutathione S-transferase M1 genotype on adduct levels. Transplacental transfer of hydroxyethylating agents was also studied by the analysis of umbilical cord hemoglobin. The adduct levels in smokers were significantly higher than those in nonsmokers. The adduct levels in umbilical cord blood globin were quantitatively related to those in maternal blood (maternal:fetal ratio 2.7 in smokers and 2.8 in nonsmokers). In the nonsmokers, there was no statistically significant difference in the adduct level between the urban and rural areas, but the level in suburbia tended to be lower than that in the rural area. In the combined smoker and nonsmoker groups, there was no effect of the glutathione S-transferase M1 genotype on levels of N-(2-hydroxyethyl)valine.