The tanned red cell hemagglutination inhibition immunoassay has proved to be a sensitive test for the measurement of fibrinogen-fibrin degradation products (FDP-fdp). The case of a patient who initially had a negative test for FDP-fdp due to the presence of an anti-D antibody and the use of O-positive tanned erythrocytes in the hemagglutination inhibition immunoassay is presented. When a control consisting of buffer substituted for the anti-fibrinogen antiserum, tanned erythrocytes and patient's serum is used, the agglutination due to incompatibility will be recognized. The irregular antibody can then be removed by absorption, and the titer for FDP-fdp can be accurately measured.