A pair of identical twins was evaluated for menometrorrhagia and mild post-operative bleeding. Each had a slightly prolonged partial thromboplastin time and prothrombin time. Full hemostatic evaluation was normal, except that plasma prothrombin activity was 23% and 25% of normal with a Russell's viper venom assay. Each twin had 70% of normal plasma prothrombin antigen. Prothrombin kinetic data were obtained before and during hysterectomy. In these twins, infused prothrombin (Konyne) was found to have a volume of distribution of approximately one and one-half times the plasma volume and a half-life of 34 hours. The previously reported dysprothrombins are briefly reviewed. Prothrombin Gainesville is the twelfth such disorder reported.