Oxidative metabolism of estrogens has been associated with genotoxicity. O-methylation of catechol estrogens is considered as a protective mechanism. 4-Methoxyequilenin (4-MeOEN) is the O-methylated product of 4-hydroxyequilenin (4-OHEN). 4-OHEN, the major catechol metabolite of the equine estrogens present in the most widely prescribed hormone replacement therapeutics, causes DNA damage via quinone formation. In this study, estrogen receptor (ERa) binding of 4-MeOEN was compared with estradiol (E2) and equilenin derivatives including 4-BrEN using computer modeling, estrogen response element (ERE)-luciferase induction in MCF-7 cells, and alkaline phosphatase (AP) induction in Ishikawa cells. 4-MeOEN induced AP and luciferase with nanomolar potency and displayed a similar profile of activity to E2. Molecular modeling indicated that MeOEN could be a ligand for ERa despite no binding being observed in the ERa competitive binding assay. Methylation of 4-OHEN may not represent a detoxification pathway, since 4-MeOEN is a full estrogen agonist with nanomolar potency.