1993). Two studies demonstrated convincing increases in urinary levels of 3-ethyladenine in smokers, indicating the presence in cigarette smoke of an unidentified ethylating agent (Kopplin et al. 1995; Prevost and Shuker 1996). There was no effect from smoking on urinary levels of 3-(2-hydroxyethyl)adenine (Prevost and Shuker 1996). A population-based study found higher levels of both 3-methyladenine and 7-methylguanine.