BACKGROUND: Thromboplastin extracted from human brain has been replaced by other animal sources due to the risk of infection. However, there is a variation in sensitivity according to the animal it was extracted from, making the monitoring of the prothrombin time (PT) in patients on anticoagulant therapy particularly difficult. Standardization of PT is made by comparing each thromboplastin with an international reference preparation, calculating the ISI (International sensitivity index), and expressing the PT in INR (International Normalized Ratio). Thromboplastins that are currently available commercially are from rabbit brain, human placenta, or bovine brain. This study was undertaken to find the best animal source by comparing the precision of thromboplastins derived from different animals, ISI, and the sensitivities to factor VII deficeint plasma. METHODS: The thromboplastin was extracted from the animal's brain with normal saline. The precision of the thromboplastin and the sensitivity to factor VII deficient plasma were measured. Fresh plasmas from 108 healthy individuals and 69 patients on oral anticoagulant therapy were obtained to measure the PT using the thromboplastin from human or animal brain. From the results, the ISI and CV (correlation of variation) were calculated. RESULTS: The average PT for normal individuals using thromboplastins from human (HBT), rabbit (RBT), bovine (OBT), swine (PBT), and canine (DBT) brain were 13.8, 18.3, 26.7, 26.2, and 37.3 seconds, respectively. Thromboplastin from chicken brain failed to induce coagulation. The prolongation of PT with factor VII deficient plasma for HBT, RBT, OBT, PBT, and DBT were 2.96, 2.96, 2.69, 2.47, and 3.08 respectively. The ISI for RBT, OBT, PBT, and DBT were 1.37, 1.39, 1.54, and 1.20 and the CV were 0.9, 0.9, 0.82, and 1.04, respectively. CONCLUSION: The precisions were high for all thromboplastins with the exception ofDBT. The thromboplastins were all sensitive to factor VII depleted plasma. The ISI were less than 1.6 and the CV were all less than 3%. The average PT of healthy individuals was shortest using RBT and the ISI was also low (1.4), indicating that rabbit brain is a suitable source of thromboplastin.