Measurements of plasma ADAMTS13 activity (ADAMTS13:AC) have been used for the diagnosis of patients with thrombotic thrombocytopenic purpura (TTP); however, the clinical usefulness of plasma ADAMTS13 antigen (ADAMTS13:AG) has been controversial, because antigen values vary widely among patients with acquired idiopathic TTP (ai-TTP). We have developed a novel enzyme-linked immunosorbent assay (ELISA) for the determination of plasma ADAMTS13:AG. This highly sensitive ELISA system using a neutralizing monoclonal antibody enables the detection of as little as 0.1% of the level in normal human plasma, corresponding to approximately 1 ng/mL purified plasma ADAMTS13. The mean (+/- 2 SD) plasma level of ADAMTS13:AG in healthy individuals was 106.4% +/- 39.3% (n = 52). Patients with Upshaw-Schulman syndrome (USS) (n = 20) and ai-TTP (n = 30) showed significantly reduced ADAMTS13:AG levels (0.5% +/- 1.6% and 1.2% +/- 3.4%, respectively). The ADAMTS13:AG level was 48.4% +/- 42.6% in USS carriers (n = 40) and <8.3% in ai-TTP patients with <0.5% ADAMTS13:AC. These values were almost parallel to those for ADAMTS13:AC. This ELISA may be useful for the rapid determination of ADAMTS13:AG. Further investigations of this antigen would be helpful in advancing the understanding of the pathogenesis of congenital and acquired TTP.