To gain a better understanding of the anticomplement activity of triterpenoids, the complement activity of five unusual oleanane-type triterpenoids, bearing a carboxyl group at the C-29 position, were estimated against the classical pathway of a complementary system. The five triterpenoids were obtained from the roots of Aceriphyllum rossii (Saxifragaceae), and were determined to be aceriphyllic acids B-E (1-4) and K (5). Of the isolated compounds, compounds 1, 2 and 5 showed anticomplement activities, with $IC_{50}$ values of 328.4, 77.5 and 348.6 ${mu}M$, respectively, whereas, compounds 3 and 4 were inactive. This showed that a carboxyl group at the C-23 position and an ${alpha}$-configuration of a hydroxyl group at the C-3 position in the olean-29-carboxylic acid triterpenoids seemed to play an important role in the anticomplement activity of human serum against erythrocytes.