Rhamnan sulfate was extracted with boiling water from the cell wall of Monostroma nitridum, and the resulting extract was purified by ion-exchange and size-exclusion chromatography. The polysaccharide, which is regarded as a homopolysaccharide, was 6-fold more antithrombin-active than the heparin standard. The antithrombin activity was decreased, by desulfation, although the non-active product still retained 8% of the sulfate ester. A comparative structural study of the intact rhamnan sulfate and rhamnan, its desulfated product, by periodate oxidation, Smith degradation and methylation analysis revealed the rhamnan sulfate to consist of alpha-1,3-linked L-rhamnose residues, some of which were substituted with sulfate groups mainly at position O-2. Minor amounts of internal 1,2-linked rhamnose and branched rhamnose linkages were also detected.