The regional structure of fibrinogen was considered in reference to the terminal plasmin-cleavage fragments, D and E. By two independent approaches, a yield of two D and two E fragments was determined and this established the presence of two D and two E regions in each fibrinogen molecule. Immunochemically, it was shown that the expression of native fibrinogen determinants by the D:E complex was fully reconstituted by an equimolar mixture of D and E fragments, while other recombinant ratios failed to yield optimal reconstitution. Utilizing the cleavage-associated neoantigen of fibrinogen, fg-D(neo), as a quantitative immunochemical marker, it was determined that complete digestion of fibrinogen by plasmin yielded two D fragments. Since the D:E complex appeared to consist of an equal number of D and E fragments, the presence of two E regions and two D regions in fibrinogen was indicated. In polyacrylamide gel electrophoresis, the linear relationship between the concentration of the D or E fragment and its densitometric area in the gel was utilized to quantitate the yield of D and E fragments in terminal digests. The yield of two D and two E fragments was demonstrated and it was also shown that the yield of fragments was independent of the plasmin concentration and of the length of exposure of the D and E fragments to the enzyme. Thus, it appears that fibrinogen is a highly symmetrical molecule consisting of two E regions as well as two D regions.