A protein having affinity to lipopolysaccharide of Escherichia coli K12 was purified to homogeneity from the hemolymph of Periplaneta americana. This protein, with an average molecular mass of 450 kDa. was a homooligomer of a 28-kDa subunit protein. Comparative studies using lipopolysaccharide molecules of E. coli and Salmonella minnesota suggested that this protein recognizes and binds to a specific carbohydrate structure of E. coli lipopolysaccharide. Ca2+ was required for this protein to bind to lipopolysaccharide, but other divalent cations could not replace Ca2+.