The aim of the study was to settle a concent for the standardised description of the potency of a PCR setup with respect to the detection limit. The main terms used are the theoretical detection limit (L theor ) and the practical detection limit (L prac ). These detection limits were determined for the example of two current PCR setups for the detection of genetically modified soybeans and maize. The L theor for the detection of Roundup Ready soybean DNA with the PCR setup applied was 0.005% genetically modified organism (GMO)/non-GMO (w/w) (corresponding to 30 copies of the GMO soya genome per single PCR reaction). In pre-mixed powder preparations (certified reference materials, CRMs) of soya it was possible to detect 0.1% GMO/non-GMO (w/w), i.e. this was the L prac . The L theor for Bt maize was 0.005% GMO/non-GMO (w/w), that corresponded to 9 calculated genome copies per single reaction. Without background DNA, as few as 2 genome copies (0.01 ng GMO DNA) were clearly detectable. As for soybeans, the L prac of the available CRMs from transgenic and conventional maize was 0.1% GMO/non-GMO (w/w). Quantitative systems for the investigation of foodstuffs are still not available as official methods, and are still being developed. Determining L theor and L prac values could be important for setting limits for the GMO contents of foods, when deciding whether they should be labelled. Concerning labelling limits, it is obvious that the PCR setups investigated have more sensitive detection thresholds than the limits under discussion for the labelling of foodstuffs as "genetically modified".