A direct enzyme-linked immunosorbent assay (ELISA) employing 2/215 mouse monoclonal hybridoma antibody is described. The assay is capable of detecting activated factor XII in human plasma and can be used to assess early detection of the intrinsic blood coagulation pathway. No cross reactivity with human factor XII zymogen has been found. The assay was used to assess activation of factor XII in patients with renal failure, pregnancy and diabetes compared to a control group.