The aim of this study was to introduce the use of a peptide nucleic acid (PNA)-thiazole orange conjugate for real-time monitoring of PCR. When the so-called light-up probes hybridize sequence-specifically to the PCR product, an increase in the fluorescent signal is obtained. It was found that the light-up probe can quantitatively measure the amount of DNA or intact bacterial cells in the reaction mixture, without interfering with the PCR amplification. A linear detection range of at least 4 log units was obtained without optimization of the system. The detection limit of this light-up assay per reaction mixture was 0.4 pg genomic Yersinia enterocolitica DNA.