There is DNA contamination somewhere in your PCR assay system. Use only PCR-grade reagents and lab ware. Wear gloves throughout the procedure. Always use fresh pipette tips, water and other reagents. Do not leave lab ware (open tubes and tip boxes) exposed to the air for long periods of time. The most common source of DNA contamination comes from the PCR products of previous experiments. Avoid the spread of any PCR products into the air of your working environment. Close all tubes containing PCR products once you are finished adding or removing volumes. Discard all tips or tubes that have been in contact with PCR products into a container of bleach. Clean your bench and your pipettors often. Some researchers expose lab ware with UV light to render any contaminating DNA ineffective in PCR through the formation of thymidine dimers.