Two novel biologically active short peptides, aeruginosins IN608 and IN652, were isolated from the cyanobacterium Microcystis sp. strain BHU006, which was collected from Durgakund water reservoir in Varanasi, India. Aeruginosins IN608 and IN652 are linear modified peptides containing four building blocks, one of which is the arogenate-derived modified amino acid 2-carboxy-6-hydroxyoctahydroindole. Aeruginosin IN608 and aeruginosin IN652 inhibit the activity of the proteolytic enzyme trypsin with IC50's of 4.3 and 4.1 μM, respectively.