We developed a new rapid assay for the factor XIa-alpha 1 antitrypsin complex (FXIa-alpha 1AT) in plasma using an anti FXI monoclonal antibody (KMXI-1). In 20-fold diluted plasma samples, this assay was not affected by co-existing FXI or non specific color development of the plasma. Normal level of FXIa-alpha 1AT (11 +/- 4.1 ng/ml plasma) increased with the aging of healthy adults. The FXIa-alpha 1AT levels of patients with disseminated intravascular coagulation (DIC) rose along with the progression of the disease, and the appearance of high levels and the peak of FXIa-alpha 1AT developed faster than FDP-E or alpha 2plasmin inhibitor-plasmin complex (alpha 2PI-PmC) in most patients. These results indicate that, in addition of FDP and alpha 2PI-PmC, FXIa-alpha 1AT is a useful molecular marker for DIC.