A reversed passive hemagglutination technique in which the antibody of fibrinogen is used as a model and the tannic acid is used as a coupling reagent for the attachment of antihuman fibrinogen serum to erythrocytes is described. Various factors which influence the coupling of purified rabbit antihuman fibrinogen serum IgG were examined. A standard sensitizing procedure was deduced. It is simple and rapid, and can also be used for coupling of other proteins and testing their antisera and antigen. It is superior to passive hemagglutination inhibition test for estimation of FDP. Through laboratory observation and clinical use for two years, it is considered that this method is simple, sensitive and the results are reproducible. It provides a useful tool for the analysis of FDP.