A purified thermostable nucleic acid polymerase is obtained that has unique characteristics. Preferably the nucleic acid polymerase is DNA polymerase isolated from a Thermus aquaticus species and has a molecular weight of about 86,000-95,000 daltons. The thermostable nucleic acid polymerase may be native or recombinant and may be used in a temperature-cycling chain reaction wherein at least one nucleic acid sequence is amplified in quantity from an existing sequence with the aid of selected primers and nucleotide triphosphates. The nucleic acid polymerase is preferably stored in a buffer containing non-ionic detergents that lends stability to the nucleic acid polymerase. A preferred buffer contains glycerol, polyoxyethylated sorbitan monolaurate, ethoxylated nonyl phenol and gelatin.