During batch cultivation, the cyanobacterial strain Synechocystis aquatilis Sauvageau B90.79 was found to release a fucose-rich sulfated polysaccharide, which was mainly composed of arabinose (45%) and fucose (47%) and had a degree of sulfation of 0.43. The released polysaccharide showed merely an anticoagulant activity of less than 10% compared to the reference compound, unfractionated heparin. However, distinctive effects on the complement activation were observed: its inhibitory effect on the classical pathway of complement activation was 600-fold stronger than that of unfractionated heparin, whereas that on the alternative pathway of complement activation was 2- to 3-fold weaker. The results indicate that this biotechnologically producible, released polysaccharide represents a specifically acting complement modulator.