Lipoprotein(a) [Lp(a)] is a low-density lipoprotein (LDL)-like lipoprotein particle recently described as a risk factor for premature coronary heart disease, stroke, and atherosclerosis. Structurally, Lp(a) is similar to LDL in that it has comparable lipid composition and contains apolipoprotein B-100 (apo B-100). In addition, Lp(a) contains the glycoprotein apolipoprotein(a) [apo(a)], which is disulfide-linked to apo B-100. The recent awareness of a striking correlation between atherosclerosis and concentrations of Lp(a) in plasma prompted our development of an accurate quantitative assay for plasma Lp(a), a monoclonal-antibody-based enzyme-linked immunosorbent assay for Lp(a) that is shown to be sensitive, precise, and highly specific. The response to several isoforms of Lp(a) is linear, and as many as 80 samples can be quantified on one plate. This easily performed assay is suitable for use in the clinical laboratory and for screening large populations.