In states of plasmic hypercoagulability and consumption coagulopathy ethanol favours the non-enzymatic polymerization of circulating soluble fibrinogen fibrin monomer complexes (FFMC) in vitro. The ethanol-gelation test of Godal and Abildgaard makes use of this phenomenon, called paracoagulation. The present studies show that it is also possible to visualize soluble FFMC by means of ethanol-gelation. In the electron microscope, FFMC, polymerized non-enzymatically by ethanol in the spleen, are characterized by plump or slender mycelioid fibrillar precipitates that show a uniform rhythmic transverse striation, a period-coincidental filamentary arrangement and an average periodicity of 23 nm. The ultrastructure demonstrates these ethanol-induced filaments to be in vitro-polymerized fibrin monomer derivatives. Paracoagulation with ethanol allows the identification of soluble FFMC in the tissue prior to the formation of highly polymerized fibrin-rich microthrombi, the established equivalents of the DIC-syndrome. The electron microscope studies also show the existence of a second type of fibrillary structure in the tissue polymerized by ethanol. This second type lacks the characteristic periodicity of fibrin and the period-coincidental arrangement of the filamentary structures, but is characterized by closely packed or chain-like aligned, irregularly sized spherical bodies. There is some evidence that these spherical bodies in vitro represent non-enzymatically polymerized complexes of fibrin monomers and fibrin degradation products (FDP), the equivalent of a limited local or generalized fibrinolysis in vivo.