Levels of hypocoagulabity induced with coumarin derivatives (aconocoumarin) were determined in 10 patients Quick's test was used to calibrate commercially available thromboplastin reagents against a reference reagent. The results were then standardised in the form of a correct prothrombin ratio. It was found that comparable results could be obtained, even with poorly sensitive thromboplastins, by determining a thromboplastin sensitivity index and using it to convert the prothrombin ratio obtained with each thromboplastin into a "correct" ratio, i.e. one that allowed for the depressed sensitivity of the thromboplastin concerned. It was found that thrombotest was more sensitive than the commercial thromboplastins; it was also superior to the reference reagent in expressing the level of drug-induced hypocoagulability. Attention is drawn to the soundness of expressing prothrombin times in terms of the correct ratio to cut out the effect of variations in sensitivity on the part of commercial thromboplastins. At the same time, stress is laid on the superiority of thrombotest in the monitoring of oral anticoagulant therapy.