Plasmic degradation of crosslinked fibrin has been studied to identify the proteolytic cleavages that convert the clot into a soluble lysate and also to identify the derivatives that are likely to circulate during clot dissolution. Initial polypeptide chain cleavages do not disrupt the solid clot matrix. With continued exposure to plasmin, high molecular weight derivatives are produced that remain attached to the clot by noncovalent forces. Further degradation then results in the liberation into solution of several large, noncovalently bound complexes. Progressive degradation of the largest, initially liberated complexes to the terminal derivatives, DD/E, DD, and E, occurs in solution after their release from the clot. As the fibrin clot is exposed to plasmin for longer intervals, progressive dissolution occurs, but the structure of the covalently bound insoluble fibrin core, the noncovalently attached derivatives, and the liberated complexes remains constant. Since much of the initially liberated protein is in complexes larger than DD/E, these derivatives probably represent the more prevalent plasmic degradation products of crosslinked fibrin in vivo.