- An asymptomatic 50-year-old male with a gamma globulin paraprotein was found to have prolonged prothrombin time, activated partial thromboplastin time, and thrombin time but a normal reptilase time. The prolonged clotting times were not the result of a factor deficiency because they were not corrected by the addition of normal plasma. Instead, this patient had an antibody that delayed thrombin-mediated fibrinopeptide B release thereby producing an apparent dysfibrinogenaemia. His isolated IgG prolonged the thrombin clotting time of both normal plasma and fibrinogen. Precincubation of his IgG with fibrinopeptide B, but not with fibrinopeptide A or thrombin, decreased its ability to prolong the thrombin clotting time. The patient's purified IgG but not control IgG delayed thrombin-mediated fibrinopeptide B release from fibrinogen without affecting the release of fibrinopeptide A. These studies define a novel, clinically silent dysfibrinogenaemia due to an antibody that delays thrombin-mediated fibrinopeptide B release from fibrinogen thereby markedly prolonging the clotting times.