PCR primers artifact - (Aug/20/2007 )

hi, I'm performing some PCR with a new primers couple (24 nucleotides each). When I run the PCR product using an agarose gel, I can see a faint band of about 450 bp in all samples included the negative control (without DNA). The problem is that it is not a proper band but it stay in the middle of the well and has a smile shape. Do you think it is a contamination or a primer artifact? My doubt is that 400bp is a lot for a primer artifact.
Thanks

sorry, but you have a contamination!!! 400's to big for primers dimers....check the water, primers and dNTP's. make sure that the tubes, tips and pipettes are clean.

I also think its contamination.