Eight new linear peptides, microginins 478 (1), 51-A (2), 51-B (3) and 91-A (4) to 91-E (8), congeners of microginin, were isolated from the cyanobacterium Microcystis aeruginosa. These structures were elucidated to be 1±8 on the basis of 2D NMR data and chemical degradation. These peptides inhibited aminopeptidase M (1, 2 and 6±8) and angiotensin-converting enzyme (1). The structure± activity relationship within a series of degradation and derivative products of microginin was investigated by the chemical studies.