Serum mannan-binding protein (MBP), a lectin specific for mannose and N-acetylglucosamine, was revealed to activate the complement system as measured by passive hemolysis using sheep erythrocytes coated with yeast mannan. In contrast, rat liver MBP, which shares many properties in common with serum MBP, could not activate complement at all. The activation by serum MBP was inhibited effectively by the presence of haptenic sugars and dependent absolutely upon the presence of C4, indicating that the activation is initiated by the sugar binding activity of MBP and proceeds through the classical pathway. The 25 NH2-terminal amino acid sequence of rat serum MBP determined in this study was completely matched with that of MBP-A deduced from cDNA sequence by Drickamer et al. (Drickamer, K., Dordal, M. S., and Reynolds, L. (1986) J. Biol. Chem. 261, 6878-6887), revealing that MBP-A is in fact identical with serum MBP. On the basis of the knowledge of primary structures and physicochemical properties of rat serum and liver MBPs, a possible mechanism of the complement activation by serum MBP is discussed with reference to close similarity in the gross structures of serum MBP and C1q.