We quantitatively assayed the levels of cross-linked fibrin degradation products (D dimer) in plasma at 73 points in time in 32 patients with elevated fibrin/fibrinogen degradation products (FDP) in serum. The assay of FDP was performed on serum samples prepared in test tubes containing 5 U/ml thrombin (final concentration) by a method based on latex agglutination using antifibrinogen antibodies, and the levels of D dimer in plasma were determined by a newly developed latex immunoassay using monoclonal antibodies which do not cross-react with fibrinogen. 5 patients with chronic myelogenous leukemia (CML) had highly elevated FDP levels with normal levels of D dimer. Plasma samples from such patients with CML were treated with various concentrations of thrombin (2-10 U/ml) and after the removal of fibrin clots the levels of FDP in supernatants were assayed by the FDP assay procedure described above. The levels of FDP were normalized when plasma were treated with 10 U/ml thrombin. In 2 patients with CML who had elevated levels of FDP in serum, it was impossible to remove fibrinogen completely by addition of 5 U/ml thrombin. However, FDP levels in the sera treated with 5 U/ml thrombin were almost normal in normal controls and patients with other diseases than CML. From these results it is concluded that residual fibrinogen reacted in the assay procedure as markedly increased FDP in supernatants and elevated FDP levels in serum reflected fibrinogen-related materials, which may not completely polymerize in the presence of lower concentrations of thrombin in some patients with CML. The assay of D dimer in plasma using monoclonal antibodies is recommended in cases of CML to rule out disseminated intravascular coagulation.