Human lipoprotein [a] contains at least two high-molecular-weight, disulfide-linked apolipoproteins, apo[a] and apo B-100. Apo[a] is a highly glycosylated, hydrophilic apoprotein that somewhat resembles plasminogen by containing an extended kringle domain and a carboxyl-terminal serine protease domain. The apo [a] kringle domain is composed of 11 distinct kringle types. Ten of these display high sequence homology to plasminogen kringle 4 (PGK4). The crystallographic coordinates for PGK4 were used to generate three-dimensional molecular models of the apo[a] kringle types, and the lysine-binding region of PGK4 was used to compare the different potential receptor-ligand and ligand-binding sites contained in each different PGK4-like kringle of apo [a]. A receptor-ligand site can be proposed for each kringle type. Potential serine protease cleavage sites, containing arginine-threonine and threonine-arginine, are located on the surface of the kringles. The ligand-binding site of one apo [a] kringle model is almost identical to that of PGK4 and may be a lysine-binding site of apo [a]. Four other apo [a] kringle models appear to have structurally similar lysine-binding sites, but with differences that may influence ligand-polypeptide specificity. Five apofa] kringle models have ligand-binding sites that probably do not bind lysine; one of these is the highly repeated kringle in the known apo [a] polymorph.