An analysis of the one-stage prothrombin time using thromboplastins highly (Thrombotest®) and slightly (Normotest®) sensitive to the Pivka (protein induced by vitamin K absence) inhibitor is presented. A one-tenth dilution of blood with 0.1 mol/l Na3 citrate was satisfactory even at an extreme packed cell volume (PCV). Whole blood and plasma were equally satisfactory as test materials; however, their accuracy and precision were greatly improved when a small sample size at a high coagulation activity and a large sample size at a low coagulation activity were used. The PCV correction method which corrected for the different Pivka inhibitor sensitivities of thromboplastins was preferabl