As shown in several screenings, cyanobacteria (blue-green algae) are sources of very interesting metabolites, many of which possess significant biological activities . Cyanopeptides, as a rule, metabolites of a peptidic nature, show in addition to hepato- and neurotoxic properties, a broad spectrum of biological activities, including antitumour , immunosuppressive  and antimicrobial effects , as well as angiotensin-converting enzyme inhibitory action  and cardioactive effects . Many of the isolated, non-toxic compounds inhibit serine proteases, which play a central role in the human organism. Trypsin-like serine proteases (e.g. thrombin, plasmin, factor Xa, t-PA, or tryptase) are the leading factors in blood coagulation, fibrinolysis, kinin-kallikrein and complement systems as well as in inflammatory processes. Both the kinin-kallikrein and complement systems are thought to be closely related to inflammation and immune reactions. Failures of one or more of these enzymes may cause a state of imbalance between protease and antiprotease (endogenous protease inhibitors) and may lead to an excess of proteolytic activity and to the development of diseases such as thrombosis, heart failures, further thromboembolic events, asthma, multiple sclerosis and pancreatitis. The discovery and development of oral inhibitors of the above mentioned enzymes therefore presents a notable measure for improving the treatment of these disorders and remains a challenge for each medicinal chemist. This paper reviews the low-molecular weight, serine protease inhibitory cyanopeptides published over the last decade and reports on actual efforts and developments in establishing structure-activity relationships concerning the inhibition of trypsin-like serine proteases.