For the elucidation of the biosynthetic relationship between non-toxic and hepatotoxic peptides produced by cyanobacteria, we compared the secondary metabolites from the toxic and non-toxic cyanobacteria, Oscillatoria (Planktothrix) agardhii. Three groups of peptides, cyclic depsipeptides having a 3-amino-6-hydroxy-2-piperidone moiety, a cyclic nonapeptide and cyclic peptides possessing an ureido linkage were isolated together with microcystins from the toxic O. agardhii strains 97 and CYA128. A tricyclic peptide composed of 14 amino acid residues was isolated from the non-toxic O. agardhii strains 2 and 18. (C) 2000 Elsevier Science Ltd.