The concept that lipoprotein (a) [Lp(a)] is a surrogate for ascorbate is suggested by the fact that this lipoprotein is found generally in the blood of primates and the guinea pig, which have lost the ability to synthesize ascorbate, but only rarely in the blood of other animals. Properties of Lp(a) that are shared with ascorbate, in accordance with this hypothesis, are the acceleration of wopnd healing and other cell-repair mechanisms, the strengthening of the extracellular matrix (e.g., in blood vessels), and the prevention of lipid peroxidation. High plasma Lp(a) is associated with coronary heart disease and other forms ofatherosclerosis in humans, and the incidence of cardiovascular disease is decreased by elevated ascorbate. Similar observations have been made in cancer and diabetes. We have formulated the hypothesis that Lp(a) is a surrogate for ascorbate in humans and other species and have marshaled the evidence bearing on this hypothesis.Lipoproteins consist of particles, each of which is a globule of lipid molecules surrounded by an apoprotein shell. Lipoprotein(a) [Lp(a)] was discovered by Blumberg et al. (1) and by Berg (2). It shares with low density lipoprotein (LDL) its lipid and apoprotein composition-mainly apoprotein B-100 (apo B), consisting of a polypeptide chain of 4536 amino acid residues. The unique feature of Lp(a) is an additional glycoprotein, designated apoprotein(a) [apo(a)], which is linked to apo B by disulfide groups. The cDNA sequence of apo(a) (3) shows a striking homology to plasminogen, with multiple repeats of kringle 4, one kringle 5, and a protease domain. The isoforms of apo(a) vary in the range of 300 to 800 kDa and differ mainly in their genetically determined number of kringle 4 structures (3); apo(a) has no plasmin-like protease activity (4), but a serine protease activity has been demonstrated recently (5). Like plasminogen, Lp(a) has been shown to bind to lysine-Sepharose, immobilized fibrin and fibrinogen (6), and the plasminogen receptor on endothelial cells (7)(8)(9). This binding is inhibited by E-aminocaproic acid, certain other amines, and plasminogen.