Recently, monoclonal antibody (DD-3B6) to fibrin D-dimer was prepared and coupled to latex beads to provide a specific test (Dimertest) for fibrinolysis. The purpose of this study was to evaluate the Dimertest assay as a clinical laboratory test for the measurement of plasma fibrin D-dimer derivatives. The Dimer-test assay specifically detected 2 micrograms/mL of purified fibrin D-dimer or fibrin D-dimer/fragment E complex added to afibrinogenemic plasma but did not detect 500 micrograms/mL of either fibrinogen fragments X, D, E, or 160 micrograms/mL cross-linked fibrinogen. The fibrin(ogen) degradation product (FDP) assays of American Dade or Wellcome Diagnostics detected 5.0 micrograms/mL of fibrin D-dimer and from 1 to 10 micrograms/mL of the other FDPs. Twenty-eight percent of 150 random plasma samples assayed from hospitalized patients were positive for fibrin D-dimer derivatives. Plasma samples from 152 patients suspected of having disseminated intravascular coagulation (DIC) were assayed for serum FDP (Wellcome Diagnostics) and plasma fibrin D-dimer derivatives. Samples from 69% of patients with serum FDP levels less than 10 micrograms/mL, and more than 90% of those with serum FDP levels greater than 10 micrograms/mL, were positive for fibrin D-dimer derivatives. Dimertest results were not modified by heparin, streptokinase, freeze-thawing, or clotting plasma. Serum fibrinogen-related antigens were immunoadsorbed from Dimer-test positive sera by anti-fibrinogen antibody and formalin-fixed Cowan I strain Staphylococcus aureus. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and protein blotting with the use of monoclonal antibody DD-3B6 demonstrated a protein band with similar mobility to purified D-dimer. The measurement of plasma fibrin D-dimer derivatives by the Dimertest assay is a rapid, sensitive, and specific laboratory test for fibrinolysis. The Dimertest assay has proven to be a useful addition to the clinical laboratory and should be helpful in the diagnosis and management of patients with diseases associated with fibrinolysis.