A new system for the detection of fibrin degradation products in whole blood has been developed. The test provides a clearly visible agglutination of the patient's red blood cells in the presence of elevated levels of the crosslinked fibrin derivative, D dimer. The test, which uses a bispecific reagent prepared from Fab' fragments of monoclonal antibodies, gives a positive result in 1-2 minutes. One monoclonal antibody (RAT-1C3/86) was raised against human red blood cells, and the second (DD-3B6/22) was specific to the crosslinked fibrin derivative, D dimer. Addition of the bispecific reagent to a drop of patient's whole blood resulted in red blood cell agglutination when elevated levels of D dimer were present in the sample. Clinical trials showed sensitivity equivalent to that of current commercial tests. Samples from patients with thrombotic disease states as well as normals were examined. The test was compared with commercial latex agglutination and enzyme immunoassay systems and showed good correlation with the presence of elevated levels of crosslinked fibrin degradation products. This technology represents an advance which allows rapid "on the spot" whole blood analysis, for the diagnosis of thrombotic disorders.