example 2 and purified thrombin prepared according to example 7 were diluted with various formulation buffers as described in table 2 to a thrombin concentration of 600 u ml the thrombin preparation was then freeze dried according to example 8 and a quantity of the freeze dried thrombin was also subjected to a heat treatment of 80 u00b0 c for 72 hours thrombin clotting activity was determined as previously described to determine the percentage clotting activity that remained after freeze drying and subsequent heat treatment the results are shown in table 2 it can be seen from table 2 that formulation buffers comprising 20 mm tris hcl buffer at ph 7 2 with or without 20 mm trisodium citrate and or 150 mm sodium chloride resulted in a recovery of thrombin clotting activity after freeze drying of greater than 74 however large losses in activity were seen post heat treatment particularly in the absence of trisodium citrate the inclusion of sodium chloride in the formulation buffer gave rise to an intact plug of material whereas without sodium chloride the plug retracted and collapsed protein e g human albumin can also be included in the formulation at concentrations of 0 5 g l 10 g l to act as a bulking agent and improve plug structure and appearance when the formulation buffer was made acidic by using gluconic acid or mes buffered at ph 6 5 recovery of thrombin clotting activity after.