Two forms of hen ovalbumin that exist as genetic variants were compared by physical and immunochemical techniques. The two ovalbumins could be distinguished by electrophoretic mobility and by antisera that had been pretreated with heterologous antigen. Peptide ;maps' of chymotrypsin digests of the two ovalbumins revealed that three of the peptides were different. These were isolated and analysed. One form of ovalbumin (type B) contains the sequence: -Ser-Ser-Ala-Asp-Leu-Ser-Gly-Ile-Ala-Glu-Ser(Ser,Leu)- whereas the other form (type A) contains an asparagine residue in place of the aspartic acid residue. The -Asn-Leu-Ser- sequence of type A is not glycosylated. Chymotrypsin readily cleaved the leucine-serine bond in the asparagine-containing peptide, but not in the aspartic acid-containing variant.