Territrem B (TRB), a fungal metabolite isolated from Aspergillus terreus, potently and noncompetitively inhibited Electrophorus acetylcholinesterase (AChE EC 3.1.1.7), but had no inhibitory effect on horse serum butyrylcholinesterase (BtChE EC 3.1.1.8). The TRB-treated AChE did not recover its enzyme activity after either dialysis or dilution of the inhibited enzyme. The binding of [(14)C]TRB to AChE, but not to BtChE, was demonstrated. The concentrations of territrems required for 50% inhibition of AChE were: TRA 2.4 x 10(-8) M; TRB 1.9 x 10(-8) M; TRC 1.5 x 10(-8) M; TRA' 9.8 x 10(-8) M; TRB' 9.2 x 10(-8) M. Copyright 1996 S. Karger AG, Basel