Chicken egg yolk contains a mannose-binding protein that could be purified with a modification of the procedure of affinity chromatography and gel filtration used for chicken serum mannose-binding protein. The yolk protein was indistinguishable from the serum protein with respect to apparent molecular masses (169 +/- 7 kDa), subunits (74 kDa and 27 kDa, in approximately 1:1 ratio) produced after denaturation in the presence of mercaptoethanol, immunoreactivity with antibody against the chicken serum mannose-binding protein, amino acid composition, pH optimum for binding, calcium independence of binding, sugar-binding specificity, and specific-binding activity. Moreover, the chicken mannose-binding proteins cross-reacted with gamma-chain-specific antibody against chicken IgG. The binding proteins were identified as IgGs by several other criteria: identical electrophoresis pattern when subjected to reducing and nonreducing NaDodSO4/polyacrylamide gel electrophoresis, cochromatography on a Fractogel TSK HW-55(F) column, similar amino acid composition, and isolation of mannose-binding protein from purified serum IgG at a yield comparable to whole serum. These results support the notion that the mannose-binding proteins from the chicken serum and egg yolk are similar, if not identical, and are a subset of chicken IgG.