For localizing S-S bonds in the pig kidney Na+,K+ ATPase α subunit, cystine-containing peptides (V-1, VII-1, and VII-2), obtained in our previous study from the enzyme's tryptic digest, were analysed. Chemical modification of the cystine-containing peptides performed at cysteine residues involved successive alkylation, first with radioactive iodoacetic acid and then with ABD-F in the absence and presence of a reducing agent, respectively. Cysteinyl peptides were isolated by HPL,C, their amino acid sequences determined, and two disulfide bonds: Cys452 -Cys456 and Cys511 -Cys549 were localized by identification of fluorescent cysteine residues.