The sensitivity of a multiplex PCR for the virulence factors VT1, VT2 and eaeA specific for enterohaemorrhagic Escherichia coli (target cells, T) was adversely affected when non-pathogenic E. coli (non-target, NT) cells were present. In the absence of NT cells the sensitivity, obtained by decimally diluting the T cell culture, was T-10 (< 10 cfu ml-1) for eaeA and VT2, and T-5 (ca 10(4) cfu ml-1 T) for VT1 virulence factors. When approximately 10(9) cfu ml-1 NT cells (NT0 dilution) were present, the sensitivity dropped to T-1, T-3 and T-1 (ca 10(7), 10(6) and 10(8) cfu ml-1 T) for eaeA, VT2 and VT1, respectively. At NT-1 (ca 10(8) cfu ml-1 NT) or higher dilutions the sensitivity of eaeA and VT2 was the same as when no NT cells were present. In respect of VT1 the sensitivity gradually increased until at NT-4 the sensitivity was the same as when NT cells were completely absent. This work indicates that caution should be exercised when interpreting PCR results particularly when substantial non-target cell populations are suspected.