The quaternary benzophenanthridine alkaloids sanguinarine (1) and chelerythrine (2) inhibit rat liver L-alanine-:2-oxoglutarate aminotransferase (EC 2.6.1.2) activity. Nitidine (3) has no inhibitory effect. The inhibitory activity of alkaloids depends on the reactivity of the iminium bond with the nucleophilic reagent, e.g., the thiol group. The stability constants of adduct formation for thioethanol, cysteine, and glutathione with sanguinarine (1) and chelerythrine (2) are given. The mechanism of the inhibition of alanine aminotransferase activity by the alkaloids 1 and 2 is discussed.