The presence of minimal amounts of fibrinogen-fibrin intermediates in human plasma was visualized by an agglutination reaction of glutaraldehyde-treated human erythrocytes coated with purified fibrin monomers. A degree of monomer coating was established which produced erythrocytes not agglutinated by normal plasma but by plasma containing minimal amounts of soluble complexes of fibrinogen with fibrin monomers. Under standardized conditions of coating, erythrocyte concentration, temperature, pH, and incubation time, the agglutination time varied with the ratio of soluble fibrin to fibrinogen in plasma. The test was sensitive down to a soluble fibrin concentration of 0.675% of the plasma fibrinogen concentration. Early fibrinogen and fibrin degradation products (FDP) in the plasma led to a prolongation of the agglutination time at a concentration of more than 16 mg/100 ml. Late FDP in a concentration of 100 mg/100 ml did not convert a positive test to negative. The test was not affected by heparin and protamine at concentrations of up to 12.5 and 50 NIH units/ml, respectively.