We describe here conditions under which the enzymatic amplification of DNA using the polymerase chain reaction (PCR) is quantitative, even when the amplification reaction is run to saturation. DNA in the sample to analyze is co-amplified with known quantities of an internal standard, namely a DNA molecule whose sequence or length differs from that of the sample DNA by only a few base pairs. The two amplification products are detected as run-off products elongated from one or several additional labelled, primers. The ratio between the two signals provides a precise estimate of the amount of specific DNA in the sample to analyze.