Three new analogues of dolastatin 13, termed lyngbyastatins 5-7 ( 1- 3), were isolated from two different collections of marine cyanobacteria, Lyngbya spp., from South Florida. Their planar structures were deduced by a combination of NMR techniques, and the absolute configurations were established by modified Marfey's analysis of the acid hydrolyzates. The related cyclodepsipeptide somamide B ( 4), previously reported from a Fijian cyanobacterium, has also been found in one of the extracts, and its absolute stereochemistry was unambiguously assigned for the first time. Compounds 1- 4 were found to selectively inhibit elastase over several other serine proteases, with IC50 values for porcine pancreatic elastase ranging from 3 to 10 nM.