Molecular weight determination of dimeric phospholipase A2 from Vipera aspis zinnikeri venom (PLA2-I) was performed with electrospray ionization mass spectrometry (ESI-MS). PLA2-I consists of an acidic and a basic subunit (subunit A and B), which bind noncovalently and dissociate under highly acidic conditions. The protonated molecular ions of subunit A and B were measured to be 13,655.9 and 13,842.6, respectively. The complete amino acid sequence was also determined by Edman sequencing of the S-pyridylethylated derivative and its peptides derived from enzymatic digestion. Both subunit A and B consist of 122 amino acid residues and contain 7 disulfide bonds. The theoretical molecular mass calculated from the primary structure completely agree with the ESI-MS data. THe sequential homology between subunit A and B was 63.9%; however, subunit A lacks enzymatic and biological activities that are characteristic for phospholipase A2. Although the amino acid residues essential for calcium binding (Tyr28, Gly30, Gly32, and Asp49) and catalysis (Asp92) were preserved, replacement of functionally important residue (His48) for catalysis with a Gln was found in subunit A. In addition, substitution of acidic amino acid residues for basic ones and hydrophilic residues for hydrophobic ones were observed in subunit A. Presumably, these changes in the primary structure of subunit A resulted in the loss of enzymatic activity and an increase in the binding ability with subunit B.