Abstract: A method for the amplification of a target nucleic acid is disclosed comprising the steps of reacting a nucleic acid with an amplification reaction mixture and a modified thermostable enzyme, wherein said modified thermostable polymerase is prepared by a reaction of a mixture of a thermostable polymerase and a chemical modifying reagent. The chemical modification reagent is an aldehyde, preferably formaldehyde. Essentially complete inactivation of the enzyme at ambient temperatures is achieved, with recovery of enzymatic activity at temperatures above 50° C.