Four depsipeptides (peptide lactones), called cyanopeptolins A, B, C and D, have been isolated from the cyanobacterium Microcystis sp. PCC 7806. They possess identical structures consisting of cyclic L-glutamic acid-γ-aldehyde, L-leucine, N-methyl-phenylalanine, L-valine, L-threonine, L-aspartic acid, hexanoic acid and a variable basic amino acid. This variable amino acid can be L-arginine (cyanopeptolin A), L-lysine (cyanopeptolin B), N_ε-methyl-L-lysine (cyanopeptolin C) and N_ε, N_ε-dimethyl-L-lysine (cyanopeptolin D), respectively. The L-glutamic acid-γ-aldehyde and the amino group of L-leucine form an unusual 3-amino-6-hydroxy-2-oxo-1-piperidine system. L-Threonine is connected to L-valine via its hydroxy-group forming an ester bonding. The hexanoic acid residue is attached to the N-terminal aspartic acid residue which is not a part of the ring structure. The isolation procedure of the four cyanopeptolins as well as structure elucidation are described. Amino acid analysis, GC/MS analysis, FAB-MS and several NMR techniques were used to reveal the structures.