Four new linear peptides, related to microginins 299-A (1) and 299-B (2), were isolated from the cyanobacterium Microcystis aeruginosa. Microginins 299-C (3) and 299-D (4), from M. aeruginosa (NIES-299), inhibited leucine aminopeptidase with IC50's of 2.0 and 6.4 μg/mL, respectively. Microginins 99-A (5) and 99-B (6), from M. aeruginosa (NIES-99), did not inhibit leucine aminopeptidase. These structures were elucidated to be 3-6 on the basis of 2D NMR data and chemical degradation.