An autoantibody with potent antithrombin activity whose action could be inhibited by toluidine blue or methylene blue.

A 41-year-old man with posthepatic liver cirrhosis and bleeding tendency was found to have a circulating anticoagulant with potent antithrombin activity. The thrombin time of the patient's purified fibrinogen was normal. This anticoagulant was residing in the patient's IgG fraction, and his IgG fraction could prolong the thrombin time of purified fibrinogen, suggesting that this circulating anticoagulant was an antibody.