Although bovine thrombin is commonly used in the operating room, there is evidence that exposure to bovine thrombin can result in the development of autoimmune antibodies, usually against factor V, which can lead to a profound coagulopathy. It is thought that impurities in bovine thrombin preparations are responsible for the adverse reactions in patients. Here we describe a case in which exposure to a relatively pure bovine thrombin preparation resulted in the development of an antihuman factor V antibody-associated coagulopathy. This report calls into question the safety of even relatively pure bovine thrombin.