Tryptic digest of the pig kidney Na+,K+ -ATPase was subjected to HPLC for separating water-soluble fragments. Using ammetric titration with silver nitrate, disulfide-containing peptides were identified and demonstrated to contain S-S bonds with differential sensitivity to reduction. The amino acid sequences of cystiene-containing peptides were determined by chemical modification of cysteine residues during the sequence analysis. Among the cystine containing peptides, three fragments of the α-subunit polypeptide chain were identified:Cys 452-Lys 461 Ile507-Lys519,Val545-Phe558.