A lethal DNA adduct induced by methylating agents, 3-methyladenine (m3A), is removed by both the constitutive (Tag) and inducible (AlkA) bacterial m3A-DNA glycosylases. The human 3-methyladenine-DNA glycosylase also releases m3A as well as other methylated bases. The rate of release of m3A from alkylated DNA by the purified or recombinant human m3A glycosylase is much higher than that of the other methylated bases. We now find that a partially purified recombinant human m3A-DNA glycosylase, expressed in Escherichia coli, releases at least 10-fold more 1,N6-ethenoadenine (epsilon A) than m3A from DNA. epsilon A is completely unrelated to m3A since it is a heterocyclic adduct produced by the carcinogen vinyl chloride. The rates of release of epsilon A and m3A were both dependent on protein concentration and time. The differential release of epsilon A and m3A occurs regardless of whether DNA containing each adduct is assayed separately or is assayed in a mixed substrate containing both DNAs. This result raises the question of what structural features are involved in recognition and excision by the human m3A-DNA glycosylase and what may be its primary substrate.