Conglutinin is a bovine plasma protein which mediates the agglutination of the sensitized erythrocyte-solid phase iC3b complex (conglutination). The serum mannan-binding protein (MBP) is a lectin specific for mannose and N-acetylglucosamine. Since conglutination was shown to be inhibited specifically by N-acetylglucosamine [Leon, M.A. & Yokohari, R. (1964) Science 143, 1327-1328], the possibility was raised that conglutinin might be a bovine serum MBP. The present study, undertaken to solve this problem, revealed that bovine plasma contained an MBP besides conglutinin. These two proteins were very similar in their chemical and physicochemical properties as well as binding specificity. Both bound with high affinity (Kd = 10(-8) M) to glycoproteins terminated with mannose and/or N-acetylglucosamine residues in the presence of calcium, although conglutinin preferred N-acetylglucosamine rather than mannose. They were multimeric proteins of large molecular size (over 1,000,000 daltons, and approximately 600,000 daltons for conglutinin and MBP, respectively) and consisted of a single kind of subunit with molecular weight of around 45,000. The MBP was shown to have a collagen-like structure in the molecule, as was recently reported for conglutinin [Davis, A.E., III & Lachmann, P.J. (1984) Biochemistry 23, 2139-2144]. Despite these similarities, the MBP and conglutinin were immunochemically distinct, and the MBP did not show any conglutination activity.