Stool suspensions (N = 185) prepared for routine analysis of clinical stool samples sent to the Central Virology Laboratory (CVL) at Chaim Sheba Medical Center in Israel were used to evaluate the efficiency of four different RNA extraction systems in excluding inhibitors of rRT-PCR. Ninety-three of the stool samples contained unknown amounts of rRT-PCR inhibitors (Table 1; Fig. S1 ). The remaining ninety-two stools suspensions were selected from among 860 stool suspensions archived between 2009 and 2011 that had been sent for routine enterovirus analysis (Table 1 , Figs. 1, 2, 3 and S2 ). The RNA from these archived suspensions had been extracted manually with the QIAamp Viral RNA Mini Kit and MS2 rRT-PCR and inhibition levels were known. MS2 rRT-PCR inhibitors .5 Ct were found in 93 (10.8%) of these samples. A non-random subset of 92 of these 860 samples (samples 101 to 192) with high, intermediate, and low levels of inhibitors were chosen for re-analysis so that would be a sufficient number of samples for comparison at each of these levels of inhibition.