A dysprothrombin designated prothrombin Quick, is isolated from the plasma of an individual with < 2% of normal functional prothrombin activity and 34% of the normal prothrombin level by immunologic assay. With Factor Xa or taipan snake venom as activators, a fragmentation pattern identical to that of normal prothrombin is observed on gel electrophoresis in dodecylsulfate. This evidence combined with the observed barium citrate adsorption of prothrombin Quick and the low activity suggests that the defect in prothrombin Quick is in the thrombin portion of the molecule. Thrombin Quick is isolated and comigrates with thrombin on dodecyl sulfate gel electrophoresis, either reduced or nonreduced. The activity of thrombin Quick on several biological substrates of thrombin is investigated. Relative to normal thrombin, thrombin Quick is 1/200 as active on fibrinogen and 1/20-1/50 as effective in activating Factors V and VIII and aggregating platelets. A complex with antithrombin III is detected by dodecyl sulfate gel electrophoresis. Further investigation with the active site titrant, dansylanginine-N-(3-ethyl-1,5-pentanediyl)amide showed that the thrombin Quick preparation has the same affinity for the titrant as thrombin, but apparently only 40% active sites per mole protein are titrable.