Activation of coumarin plasmas in glass tubes for 60 min resulted in a clear shortening of Thrombotest clotting times. Normotest clotting times were shortened too but to a much lesser extent. As a consequence the Normotest (NT)/Thrombotest (TT) discrepancy which was quite large at 0 time, became progressively smaller. This phenomenon was observed both in undiluted and 6∶10 diluted plasma. After ellagic acid activation a similar phenomenon was noted even though a less pronounced shortening of Thrombotest was noted. These data suggest that Thrombotest is very sensitive to contact phase and to factor VII activation and not to any coumarin-induced inhibitors.