The Lp(a) particle closely resembles low-density lipoprotein (LDL). However, the presence of apo(a), a highly glycosylated protein which is disulfide-linked to apo B100, differentiates Lp(a) from LDL. The primary structure of human apo(a) has been deduced from the sequence of a cloned apo(a) cDNA and revealed a striking homology to plasminogen (PLG). The apo(a) cDNA contained multiple copies of a motif which closely resembles the fourth kringle (KIV) of PLG, followed by sequences highly similar to kringle V and the protease domain of PLG. In addition to man, Lp(a) has been detected in Old World primates. Among them, the cDNA sequence of the rhesus monkey displays a mutation in the lysine binding site (LBS) of KIV-10, in which a Trp residue is replaced by an Arg, thereby rendering rhesus Lp(a) unable to bind to lysine residues. In order to compare the structure and function of Lp(a) in different species, we have cloned the C-terminal part of chimpanzee apo(a) spanning the region from KIV-3 to the stop codon. The global organization of this fragment is similar to that of human apo(a) with the presence of KV, absent in rhesus monkey apo(a). Nucleotide sequence comparison indicates a variation of 1.4% between chimpanzee and man and 5.1% between chimpanzee and rhesus monkey. The differences concerned single base changes. A Asp57→Asn mutation was detected in KIV-10; this residue is critical to the LBS of this kringle in human apo(a). Using an in vitro assay, we demonstrated that chimpanzee Lp(a) did not bind to intact or plasmin-degraded fibrin. We suggest that the Asn57 substitution in KIV-10 of chimpanzee apo(a) is responsible for this property. Consequently, the role of Lp(a) as a competitor in the fibrinolytic process cannot be explained by the binding of Lp(a) to fibrin in these species. Several other potential effects of Lp(a) in the fibrinolytic system, such as inhibition of t-PA activity, must therefore be explored.