Solutions of plasminogen-free human fibrinogen alone or (1) treated with sodium p-chloromercuribenzoate in order to inactivate factor XIII, or (2) enriched with factor XIII, cysteine and CaC12, were clotted with plasmin-free human thrombin and incubated under sterile conditions. The clots dissolved gradually within 2 days (fibrin from sodium p-chloromercuribenzoate-treated fibrinogen) to 15 days (fibrin from factor XIII-enriched fibrinogen). This proteolytic process was not affected by soybean trypsin inhibitor but was completely inhibited by hirudin. Gel electrophoresis of the thrombin digests indicated the formation of bands equivalent to bands X, Y, D and E of plasmin digests of fibrinogen. The two latter bands, whose identity was confirmed by immunoelectrophoresis, appeared at a more advanced stage of proteolysis than the corresponding bands of plasmin digests. The number of isopeptide bonds present did not appear to affect the rate of release of acid-soluble peptides. Gel electrophoresis and the rate of release of acid-soluble peptides indicated that fewer bonds are hydrolysed by thrombin at the time of the complete solubilization of the clot than are split by plasmin when fibrinogen becomes unclottable by thrombin.