Validated analytical methods are necessary in order to show compliance with labelling concerning conventionally produced food or food containing genetically modified organisms (GMOs). Methods to be used in arbitration should be collaboratively tested according to standardisation protocols. According to the Council Regulation of 1998, one of the marker components being the criterion for labelling of products from soy beans and maize is the genetically altered DNA. A qualitative polymerase chain reaction method which has recently been validated in a collaborative trial can be used for initial screening of food products in order to detect GMOs. Furthermore, this screening method has to be validated for processed food. In addition, there is a strong need for quantitative methods with a view to a threshold value for labelling and to distinguish between the source of GMOs in food products (ingredients or additives).