The primary structure of human plasminogen (HPg), diagrammed in Fig. 1, has been deduced from the nucleotide sequence of the cDNA (Forsgren et al. 1987) and genomic DNA (Petersen et al. 1990) that encode this protein, and has been directly determined by amino acid sequence analysis (Wiman 1973; Wiman and Wallen 1975; Wiman 1977; Sottrup-Jensen et al. 1978). HPg is synthesized as an 810-residue single polypeptide chain. A 19-residue leader peptide is excised during secretion, producing the mature form of HPg, which contains 791 amino acid residues (Forsgren et al. 1987).The only other known processing steps involved in production of plasma HPg are N- and O-linked glycosylation (Hayes and Castellino 1979a,b), and phosphorylation (Wang et al. 1997).