Fibrin monomers were made from dissolved clots according to three different methods [(1) method of DOnelly et al., using 1 m NaBr pH 5.2 as clot solvent; (2) method of Monelly-Berghaus et al., using 0.0167 m acetic acid to dissolve EDTA-clots; (3) method of Yudelman et al. using 3.3 m urea pH 7.4 as clot solvent]. The monomers were characterized by agarose gel chromatography, SDS-polyacrylamide gel electrophoresis (with and without reduction with 2-mercaptoethanol) and by acid polyacrylamide gel electrophoresis after sulfitolysis. Clottability and polymerization speed were compared to those of fibrinogen exposed to large amounts of thrombin (‘control monomers’). All preparations contained fibrin aggregates. These were of two different types, either non-covalently linked or covalently but not FSF-cross-linked aggregates. The latter type was probably due to -S-S- bonds, since they could be prevented by iodoacetamide. Methods 1 and 2 produced monomers which did not clot at all in buffer but clotted slowly in serum. Monomers prepared according to method 3 clotted even faster than did fibrinogen exposed to large amounts of thrombin. This was apparently due to the content of substantial amounts of covalently linked fibrin aggregates. When these were prevented by adding iodoacetamide under preparation, the monomers so produced had lower clotting speed than ‘control monomers’. From the present investigation it must be concluded that extrapolation of data obtained with fibrin monomers prepared from fibrin clots should be interpreted with gre