The isolation and structure of cyanopeptolin 1020 (hexanoic acid-Glu-N[-O-Thr-Arg-Ahp-Phe-N-Me-Tyr-Val-]) from a Microcystis strain is reported. Very potent picomolar trypsin inhibition (IC50 = 670 pM) and low nanomolar values against human kallikrein (4.5 nM) and factor XIa (3.9 nM) have been determined for cyanopeptolin 1020. For plasmin and chymotrypsin, low micromolar concentrations were necessary for 50% inhibition. Cyanopeptolin 1020 was found to be toxic against the freshwater crustacean Thamnocephalus platyurus (LC50 = 8.8 mu M), which is in the same range as some of the well-known microcystins. These data support the hypothesis that cyanopeptolins can be considered as a second class of toxins in addition to the well-established microcystins in Microcystis.