The polysaccharide B3-PS1 was extracted and purified from Herba Scutellariae Barbatae (Scutellaria barbata D. Don), through bioactivity-guided fractionation. Average molecular weight of B3-PS1 was about 1,700,000 Da with a composition of Gal, Glc, Man and Ara in the ratio of 4.3:1.6:1.1:1.0, and trace of Rha, Fuc and Xyl. Preliminary data showed that 1) B3-PS1 inhibited complement activation of the classic pathway with CH(50) value of 0.12 +/- 0.02 mg/ml and of the alternative pathways with AP(50) value of 0.36 +/- 0.05 mg/ml; 2) B3-PS1 interacts with C1q, C1r, C1s, C2, C3, C4, C5 and C9 through a hemolytic assay. These results strongly suggested that B3-PS1 could be a potential candidate in treating those complement-associated diseases such as rheumatoid arthritis, Alzheimer's disease, and Adult Respiratory Distress Syndrome (ARDS).