With its broad effective range for fluorescence detection, real-time PCR is one of the most valuable techniques for quantitation in molecular biology. A modified real-time PCR assay is described for determining viral load. The assay uses fluorescence to measure the number of PCR amplicons by monitoring the Invader reaction in four steps in the thermal cycle. The Invader reaction with its cleavase was performed at moderate temperature after the amplicon was denatured at a high temperature. The method was as effective as real-time PCR with a TaqMan probe in determining the quantity of virus in samples of human papillomavirus type 16. Importantly, the assay allows the use of a common probe for multiple reactions. Thus, this method is a rapid inexpensive assay with a common fluorescence probe that does not depend on the conformation of the target DNAs.