Conclusions And SummaryThe Thrombotest reagent shows a remarkable buffering capacity at the normal blood pH and shares its electrolytic composition (Na, K, CI) except for the absence of bicarbonate. The activity of the clotting factors I, V and VIII is high in contrast with a moderate Fe(II) and Cu (II) content. The stability of the reagent in suspension is excellent at - 30°C (at least during 30 days) and good at +4°C (maximum during 7 days); the Thrombotest rea-gent can hence be stored and avoids occasionally the need of a control blood sample. The test has to be performed strictly at 37°C because of its sensitivity to minor temperature differences. The reagent is easily brought into suspension, the results obtained do not differ significantly if the CaCl2solution is between 3.3 and 5.5 mM/1. The test is easily standardized since the dilution fluid used has not to be defined so strictly as for the usual commercialized tissue thromboplastin. The Thrombotest is a real progress if capillar blood is used. Activation phenomena in citrated blood are still minimized problems in the use of Thrombotest on stored blood or plasma. One of the advocated advantages of the Thrombotest method would be its sensitivity to factor IX and hence to the intrinsic clotting system. However, this last point is not completely proved and stimulates further investigations.