Ovalbumin isolated from eggs of the Japanese quail, C. c. japonica, was subjected to limited proteolysis by subtilisin to give plakalbumin and then fractionated on Sephadex G75 in acid-urea to give plakalbumin S-protein and S-peptide. The plakalbumin peptide was recovered, oxidized with performic acid, and the sequence of amino acids determined from the peptides formed by enzyme digestion. There were two cysteine residues in the 33-residue sequence. The ovalbumin was also oxidized with performic acid and digested with thermolysin and pepsin before isolating, from a sulfonated polystyrene column, the acidic cysteic acid peptides, as well as acetylated N-terminal peptides and phosphorylated peptides, and determining their amino acid sequence. Additional peptide sequences containing cysteine or half-cystine were characterized. Quail ovalbumin was reduced and carboxymethylated with [2-14C]iodoacetic acid. Peptides containing labelled S-carboxymethylcysteine residues were isolated from thermolytic digests of the carboxymethylated ovalbumin by paper ionophoresis and chromatography. Their amino acid sequence was determined and five different sequences involving labelled S-carboxymethylcysteine residues were established. The presence of two half-cystine residues and the location of the disulfide bond were shown by blocking the cysteine residues with non-radioactive iodoacetic acid, reducing the disulfide bond and labelling the half-cystine residues with [2-14C]iodoacetic acid. After thermolytic digestion of the protein, radioactive peptides were isolated by paper ionophoresis and chromatography. These studies have thus shown that quail ovalbumin contains one cystine residue and three cysteine residues, which is one residue of cysteine less than in ovalbumin from the hen (Gallus gallus domesticus). There is strong homology in the amino acid sequences of hen ovalbumin and quail ovalbumin determined in these investigations.