A method for conducting a nucleic acid amplification reaction, said method comprising forming an amplification reaction mixture in the presence of sufficient of a pyrophosphate salt to prevent primer extension taking place, digesting said pyrophosphate salt with a pyrophosphatase enzyme (PPase), and subjecting said reaction mixture to conditions such that an amplification reaction may proceed. This can be used as a "hot start" amplification. Particular novel pyrophosphatase enzymes for use in the method are also described and claimed.