We have compared the INR obtained with three different thromboplastin reagents: one rabbit and two recombinant tissue factor thromboplastins using the same coagulometer. A preliminary study has shown that freezing of plasmas at -80 degrees centigrade causes a 6% increase in INR. We did this experiment on 57 plasma samples from patients receiving oral anticoagulant therapy, none of them receiving heparin. Results show that global prothrombin activity expressed in percentage is significantly lower with recombinant tissue factors than with rabbit thromboplastin. ISI of one of the two recombinant thromboplastins did not seem to be appropriate to our coagulometer highpointing the variability of INR due to the coagulation analyzer. This underlines the necessity for each laboratory to assess the ISI value provided by the manufacturer according to its working conditions. This study suggests that the results of prothrombin time should be exclusively expressed in INR for patients treated with oral anticoagulants.