Four protocols for extracting RNA from stool samples were compared in this study. These four commercial extraction methods are among the most commonly used in the diagnostic laboratories. None of samples 1 to 93 contained MS2 RNA. Similarly Nonove et al (14) did not report any unusual amounts of MS2 RNA that would have indicated prior presence of MS2 in their samples when they added MS2 to 106 stool suspensions at concentrations two to eight-fold less than used in this study. MS2 coliphage was thus used as a non-competitive external control because of its absence from human clinical samples. MS2 is a better external control for rRT-PCR than plasmid or phage DNA since it also measures the efficiency of the RT reaction [5, 12, 13, 14, 17, 18] . Here, the MS2 was added to the lysis buffers to minimize sample manipulation and increase the uniformity between manual and semi-automatic extraction protocols. The presence of rRT-PCR inhibitors was evaluated by extracting equal volumes of stool suspension and eluting with equal volumes of elution buffer. Extractions were performed in parallel to eliminate storage related differences, and RNA was assayed on the same rRT-PCR run to minimize analytical differences [19] . Absence of MS2 in the stool samples 1-93 was confirmed by MS2 rRT-PCR of RNA extracted with the EasyMag RNA extraction system using lysis buffer without exogenous MS2. This extraction protocol, the system that had the least amount of samples with inhibitors, was chosen to increase the chance of finding any endogenous MS2 RNA in the stools.