A sensitive method is described for the rapid detection of soluble fibrin in human plasma. It is based on the agglutination of Latex particles suspended in a solution of purified fibrin monomers and Blue Dextran 2000. Photometric registration of the decrease in optical density (OD) during agglutination allows the results to be quantitated. The δOD depends on the ratio of fibrin monomers to fibrinogen in the tested plasma rather than on the absolute amount of fibrin monomers present.