Unmodified and citraconilated OSCP of the pig heart mitochondrial H(+)-ATPase were hydrolysed by proteinase from Staphylococcus aureus V8 and trypsin, respectively. To purify the individual peptides, various types of HPLC and covalent chromatography on SH-Sepharose were used. By the automatic Edman method complete or partial amino acid sequences of the peptides obtained were determined, thus allowing for the reconstruction of the primary structure of pig OSCP. A linear antigenic determinant recognizable by A1 monoclonal antibody against bovine OSCP, was localized. Studies showed Gly43 residue (bovine OSCP) to be replaced by Ala43 (pig OSCP), which is responsible for a decrease of the affinity of the monoclonal antibody A1 to pig OSCP. Comparative analysis of primary structures of bovine and pig OSCP was carried out.