Supplementary data are available online at http://toxsci. oxfordjournals.org/. FUNDING National Institutes of Health (PHS grant ES003358); United States Department of Agriculture (grant 59-0206-9-058). Pilot Evaluation of RT-PCR/Electrospray Ionization Mass Spectrometry (PLEX-ID/Flu assay) on Influenza-Positive Specimens The PLEX-ID/Flu assay has been recently developed to enable the detection and typing of influenza viruses based on the RT-PCR/electrospray ionization mass spectrometry technology. This novel assay was evaluated for typing performance on 201 positive influenza A or B nasopharyngeal swab specimens (NPS) detected by real-time RT-PCR during the 2010-2011 season. The PLEX-ID/Flu assay detected and characterized 91.3% and 95.3% of all influenza A and B samples, respectively. All non-typeable influenza A and B specimens by the assay showed low viral loads with threshold cycle values ≥ 33. Taken together, and although our results need to be confirmed by further prospective studies, the PLEX-ID/Flu assay detected positively and gave a typing result for 93% of all NPS detected positively by real-time RT-PCR, thus suggesting a potential role for influenza virus surveillance among other techniques. The World Health Organization (WHO) Global Influenza Surveillance and Response System provides essential information on the types and variants of influenza viruses circulating worldwide. Direct detection and identification of influenza strains from clinical samples are mostly performed by using specific real-time PCR assays and, less frequently, by culture-based methods [1, 2] . Genotyping or phenotyping characterization of positive cases is performed by sequencing classical PCR amplicons (requiring multiple primer pairs), followed by phylogenetic analysis, or hemagglutinin inhibition assay, respectively.