SummaryHighly cross-linked human fibrin was digested with plasmin and the nine main components (labelled initially 1 to 9) were detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The components of greater molecular weight, bands 1 and 2, which are identified with fragments Y dimer and Y-D (270,000 and 220,000) contain three subunits which are joined by disulphide bonds which originate from the α, β and γ-γ-chains of cross-linked fibrin. They show a reaction of partial identity against anti fibrinogen fragment D-sera and anti fibrinogen fragment E sera. Bands 3, 4 and 5 correspond to three species of fragment D-dimer and have molecular weights of 175,000,160,000 and 130,000. Each of these species differs in the size of the γ-γ-chain remnants, while all maintain the cross-link between the γ-γ-chains. The remnants of a and β-chains of cross-linked fibrin remain the same in all three species. The components of electrophoretic bands 6, 7 and 8 correspond to three fragments, each with a different molecular weight and with common antigenic components against anti fibrinogen fragment D-sera; these are called D6 (90,000), D7 (75,000) and D8 (65,000) and are formed by α- and β-chain remnants of equal molecular weight and by γ-chain remnants with a different molecular weight for each fragment. The last fragment to be identified, corresponding to electrophoretic band 9 (50,000 molecular weight) corresponds to fibrin fragment E. The present report is in agreement with previous studies of plasmin digestion of highly cross-linked fibrin, but differs from them in number of fragments D-dimer which appear during the digestion.