The number of stool suspension RNA extracts with inhibitors of MS2 rRT-PCR varied between RNA extraction protocols for randomly chosen stool suspensions. The amount of inhibitors in RNA extracts for individual stool suspensions also varied according to extraction procedures. This suggests that more than one type of inhibitor may be present and that different procedures do have different proficiencies in excluding them. Stool samples extracted by the KingFisher and the NucliSENS easyMag had fewer extracts with inhibitors than the QIAamp [18, 20, 21, 22] found that procedures similar to that used by EasyMag outperformed manual extraction procedures similar to QIAgen, and that MagNA Pure and KingFisher-like procedures were intermediate. In contrast fewer did not find major differences between some of these systems [23, 24] . Results presented here further confirm that RNA extracted with magnetic bead-based systems contained fewer inhibitors than column-based systems [21] . The number of samples with inhibitors was low in the first group of samples (samples 1-93) that were used to compare RNA extraction procedures. In order to better compare the four extraction procedures, we selected from among previously tested samples, sufficient numbers of archived stool suspensions that had high, intermediate, and low levels of inhibitors after extraction by QIAamp Viral RNA Mini Kit (samples 101-192). The stool suspensions were re-extracted in parallel by all four protocols and evaluated for inhibition of MS2 rRT-PCR. As before, KingFisher and NucliSENS EasyMag semi were better than the QIAamp Viral RNA Mini Kit and MagNA Pure LC2.0 Automatic extractor.