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This work presents a method that employs ultrasonic shear waves in the determination of prothrombin time (PT) and activated partial thromboplastin time (APTT), two plasma tests, during the coagulation process. Two AT-cut quartz crystal transducers are used as transmitter and receiver, with a frequency of 2.0 MHz. During the coagulation process there are random fluctuations in the amplitude of the wave transmitted through the sample of plasma that cease once the clot is formed. The time interval during which these fluctuations occur is used to indicate the coagulation time for the plasma. The method, repeated 35 times over the same plasma sample, provided for the mean of PT and APTT 12.7 s and 23.4 s with standard deviations of 0.8 s and 2.1 s, respectively. These results are in accordance with those obtained for normal plasma by means of other clinical tests.