We have mapped the cap site of the human plasminogen mRNA by primer extension and PCR techniques and found that it is located at position -161 relative to the first ATG, 97 bases upstream to the 5' end of the previously isolated cDNA clone. Seven human hepatic and non hepatic cell lines and fresh liver cells were tested for human plasminogen mRNA expression: the liver and the liver derived HepG2 cell line represent the major site of plasminogen RNA synthesis while the other cell lines (Hep3B, HeLa, IMR, 293 CaCo and SW626) show much lower levels.