The molecular weights of derivatives obtained from chemical and enzymatic degradation of fibrinogen and fibrin support a model in which the two halves of the fibrinogen molecule are covalently linked by a set of disulfide bonds at the amino-terminal region. The 2 asymmetric cleavages caused by plasmin in the fibrinogen molecule occur according to the reactions: X leads to Y + D Y leads to E + D. The quantitative analysis of the amino-terminal amino acids in fragments D (from fibrinogen) and DD (from crosslinked fibrin) yields a total of 3.0 and 6.9 moles of amino acids per mole of protein, indicating three and six polypeptide chain structures, respectively. The data on molecular weights, polypeptide chain composition and immunologic properties of fibrinogen degradation products support the hypothesis on the asymmetric pathway of fibrinogen degradation by plasmin and the formation of two fragment D and one fragment E molecules from each molecule of fibrinogen.