In this work the intra and inter-laboratory validation of a duplex real-time PCR screening method for the detection of genetically modified (gm) plants is described.  Target DNA sequences from Cauliflower Mosaic Virus 35S promoter (P35S) and nos-terminator from Agrobacterium tumefaciens (T-nos) are amplified.  The duplex real-time PCR method is using primer and probe sequences that have already been published for the individual ('single') detection of both target sequences.  The validation showed sensitivity comparable to the single PCR standard methods.  In addition, combined with a reference gene and using reference standard material, the method can be used to semiquantitatively estimate the amount of gm plants in an unknown sample.