Abstract: The invention provides a method of determining the amount of target DNA in a sample which comprises the steps of (a) dividing the sample into a plurality of aliquots; (b) adding to each aliquot a known amount of competitor DNA which is the same as the target DNA except that it comprises a recognition site capable of being detected directly or indirectly by a labelled species whereby the ratio of target DNA to competitor DNA is different in the aliquots; (c) co-amplifying the target DNA and competitor DNA in each aliquot by the polymerase chain reaction (PCR) using at least one primer which permits immobilization of the co-amplified DNA, the annealing sites of any PCR primers used being outward of the recognition site; (d) labelling the amplified competitor DNA; (e) before or after step (d), immobilizing the co-amplified DNA where not already immobilized, via at least one PCR primer; (f) assessing the amount of label on the immobilized amplified competitor DNA in each aliquot to provide an indication of the am