The orgA gene from Salmonella enterica serovar Typhimurium is involved in promoting cellular invasion of the pathogen. Its exact role in virulence is still unclear mainly due to difficulties in understanding its complex regulation. In this study a novel competitive RT-PCR (cRT-PCR) system was developed to measure the steady-state orgA specific mRNA levels in cells under various growth parameters. Previous studies have been inconsistent regarding oxygen regulation of orgA. Using our system we found that oxygen repressed the copy levels 3.5-fold in cells grown only to logarithmic phase. Oxygen repression was not observed in cells grown to early-stationary phase, a parameter that has previously been demonstrated to be the most invasive stage of growth. The importance of NaCl in orgA gene regulation is also illustrated. Significant increases in copy numbers were observed after growth in high NaCl conditions. Measuring the steady-state mRNA levels using cRT-PCR provides an accurate insight into prokaryotic gene regulation prior to translation. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.