The high concentration of organic and inorganic groups of chemicals may occur in drinking water and inhibit polymerase activity and sequester DNA templates from the amplification reaction. Furthermore, to detect a low amount of pathogenic microorganisms in drinking water samples, large volumes of water are usually concentrated to very small volumes. This often results in concurrent concentration of the different inhibitors and increased interference with PCR (Green, 2012; Schrader, 2012). To determine the inhibitory effect of potentially inhibiting substances present in the nucleic acid preparation, it has been suggested to carry out a PCR control reactions. The inhibition can be detected by changes in the threshold cycle (Ct), which indicates that lower concentrations of DNA are being amplified. Also, analysis of the PCR product is possible through a measurement of the melt characteristics of the amplicons where the change in the melt curve demonstrate modification of the PCR product (Opel, 2009). The aim of this study was to identify if some of the chemicals abundant in drinking water from Riga have an effect on PCR assay sensitivity. In this study chlorine, humic acids and iron were examined and analysed with real-time PCR by adding different concentrations to the reaction mixture. The results of the threshold cycle (Ct) and melting curve values were compared in-between the standard curves constructed for a fecal indicator Escherichia coli.