Comparative physical and chemical data are described for the human NH2-terminal Glu-plasminogen and Lys-plasminogen forms in order to determine the exact relationship between these two types of the zymogen. The molecular weights of Glu-plasminogen and Lys-plasminogen were similar and were determined to be 83, 800 plus or minus 4, 500 and 82, 400 plus or minus 3, 300, respectively, by sedimentation equilibrium methods. The molecular weights were identical in dodecyl sulfate solutions, approximately 83, 000, by sedimentation equilibrium methods. The sedimentation coefficients, s-020, w of Glu-plasminogen and Lys-plasminogen were determined to be 5.0 S, and 4.4 S, respectively. These two plasminogen forms had different partial specific volumes, and calculations of the frictional coefficients from sedimentation coefficients and molecular weights indicated conformation differences. Glu-plasminogen appeared to be larger in size than Lys-plasminogen in acrylamide gel-dodecyl sulfate electrophoresis. The amino acid compositions of Glu-plasminogen and Lys-plasminogen, and their major isolated isoelectric forms, were found to be similar, but several amino acid residues (glutamic acid, alanine, isoleucine, phenylalanine, and lysine) were found to be significantly higher in the Glu-plasminogen forms. The derived plasmins from both the Glu- and Lys-plasminogens with an nh2-terminal Lys- heavy (A) chain were found to have identical molecular weights of 76, 500 plus or minus 2, 500, and sedimentation coefficients, s-020, w of 4.3 S.