Anti-complementary active arabinogalactan IIb-1 (AGIIb-1) and inactive arabinogalactan IIb-2 (AGIIb-2) were isolated from an extract of the root of Angelica acutiloba KITAGAWA. AGIIb-1 mainly consisted of arabinose, galactose, rhamnose and galacturonic acid in a molar ratio of 2.2 : 1.0 : 0.3 : 0.4, and AGIIb-2 mainly consisted of arabinose, galactose, galacturonic acid, and glucuronic acid in a ratio of 1.0 : 1.0 : 0.08 : 0.1. AGIIb-1 was found to form molecular aggregates as the result of self-aggregation in water. The aggregated form was found in the void volume by gel filtration using Sepharose CL-2B in water. However, AGIIb-2 did not show this behavior. When the AGIIb-1 aggregate was applied to a gel filtration column in 8 M urea or NaCl solution, the position of the peak fraction shifted to a lower molecular weight region, and the molecular weight of AGIIb-1 decreased with the increase of NaCl concentration in the gel filtration. These results suggested that the molecular aggregation of AGIIb-1 was caused by hydrogen bonding and ionic interaction and was Ca2+ ion independent. The anti-complementary activity of AGIIb-1 was seen to be dependent upon the degree of molecular aggregation, and the AGIIb-1 aggregate in water showed the highest activity. The results of methylation analysis indicated that AGIIb-1 contained a large amount of 3, 5- or 3, 4-di-O-substituted arabinosyl residues whereas AGIIb-2 contained a large amount of 6-O-substituted galactopyranosyl residues. These results suggested that the self-aggregation of AGIIb-1 was concerned in the expression of anti-complementary activity and that the higher branching structure of arabinogalactan may be involved in the formation of the molecular aggregate.