To identify apoproteins present in purified low-density lipoproteins from hen egg yolk in relation with their emulsifying properties, they have been separated by SDS-PAGE. We identified two different proteins by liquid chromatography-tandem mass spectrometry analysis of the peptides obtained by the trypsin digestion of protein gel bands. Apovitellenin I was identified as a monomer and a dimer. Its amino acid sequence was totally confirmed, and molecular mass determination by liquid chromatography-mass spectrometry showed that it did not present post-translational modifications but only a slight heterogeneity by the loss of one or two amino acids at the C-terminal part of the protein. Apolipoprotein B was identified into seven bands corresponding to fragments resulting of a processing of the hen blood apo-B protein. The identity of the fragments was determined by the observation of the sequence coverage by trypsin peptides and the sequence alignment with homologous human blood apolipoprotein B-100.