In an attempt to assay native Lp (a), the anti-Lp (a) monoclonal antibody (161E2) was produced against synthetic antigen (Arg-Asn-Pro-Asp-Val-Ala-Pro). Characteristic properties of this 161E2 monoclonal antibody were identified to have reactivity to oxidative modification (treatment with CuCl22 and lipoxygenase), but to neither LDL, plasminogen nor native Lp (a). We developed a new ELISA method to measure Lp (a) modified by oxidative stress using this 161E2 monoclonal antibody as the capture and the labelled antibody, and conducted assay of the epitope in serum using BSA-peptide (16 peptides perl molecule of BSA) as the standard. Interestingly, hypertensive patients with complications showed a significantly higher level of oxidized Lp (a) in serum than did normotensive subjects (p<0.01), whereas there was no significant difference in native Lp (a) between normotensive and hypertensive subjects. The in vivo presence of the epitope was also confirmed with use of 161E2 monoclonal antibody-based immunostaining of arteriosclerotic tissue layer by positive response. Upon analyzing peptide homology of the epitope, it was shown to have high homology (88%) to the cytoplasmic domain of human α2A-adrenergic receptor.