Lipopolysaccharide from Klebsiella pneumoniae O3, which possesses the mannose homopolysaccharide as the O-specific polysaccharide, exhibits an extraordinarily high ability to activate the human complement system. We isolated the mannose-binding protein with a Klebsiella O3 lipopolysaccharide affinity column. The protein isolated had a molecular mass of much higher than 200 kDa, and it consisted of subunits with an apparent molecular mass of 32 kDa. The NH2-terminal sequence of the 32-kDa subunits was completely consistent with a part of the amino acid sequence of human serum mannose-binding protein. In immunoblotting, an anti-mannose-binding protein monoclonal antibody was definitely reactive with the isolated protein with the higher molecular mass. The protein isolated was bound exclusively to lipopolysaccharides possessing the mannose homopolysaccharide, not to lipopolysaccharide possessing the heteropolysaccharides. Klebsiella O3 lipopolysaccharide did not exhibit a high anticomplement activity in the serum from which the mannose-binding protein was depleted. It was concluded that the serum factor that bound to Klebsiella O3 lipopolysaccharide may be mannose-binding protein and that it may play a crucial role in the strong complement activation by Klebsiella O3 lipopolysaccharide.