Stability of DNA in plasma and serum. Blood of a normal volunteer was taken into tubes, previously spiked with plasmid DNA (pGEM7Zf+) to final concentrations as indicated. Serum or plasma was isolated immediately or after 3 hours of storage at room temperature. DNA was extracted, run on a 2% agarose gel, and stained with ethidium bromide. DNA in plasma is stable whereas DNA in serum is rapidly degraded. Arrows indicate intact plasmid DNA bands.