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 cystine-containing peptides were determined by chemical modification of cysteine residues during the sequence analysis. Among the cystine-containing peptides, three fragments of the alpha-subunit polypeptide chain were identified: Cys452-Lys461, Ile507-Lys519, Val545-Phe558.