An unusual heparin (approximately 1.9 mg/g of dry tissue) was isolated from the marine italian bivalve mollusk Callista chione. Agarose gel electrophoresis showed a high content of the fast-moving heparin component (85 ± 7.6%) and 15 ± 1.3% of the slow-moving species. An average molecular mass of 10 950 was calculated by PAGE analysis. The anticoagulant properties were measured as APTT (97 ± 12.1 IU/ mg) and anti-Xa activity (52 ± 7.4 IU/mg). Structural analysis of clam heparin, performed by depolymerizing heparin samples with heparinase (EC 4.2.2.7) and then separating the resulting unsaturated oligosaccharides by SAX-HPLC, revealed the presence of low amounts of the trisulfated disaccharide [&UDelta; UA2S(1→ 4)-α-D-GlcN2S6S] and a significant increase of the disaccharides bearing nonsulfated iduronic and glucuronic acids, [→ 4)-α-L-IdoA(1→ 4)-α-D-GIcNAc6S(1→] and [→ 4)-α-L-IdoA(1→ 4)-α-D-GlcN2S6S(1→], and [→ 4)-β-D-GlcA(1→ 4)-α-D-GlcN2S6S(1→]. As a consequence, Callista chione heparin is a low-sulfated polysaccharide showing a specific decrease of the sulfatation in position 2 of the uronic acid units.