ABSTRACTNontoxigenic strains ofCorynebacterium diphtheriaerepresent a potential reservoir for the emergence of toxigenicC. diphtheriaestrains if they possessed functional diphtheria toxin repressor (dtxR) genes. We studied the predominant strain of nontoxigenicC. diphtheriaecirculating in the United Kingdom to see if they possesseddtxRgenes and ascertain whether they were functional. A total of 26 nontoxigenicC. diphtheriaestrains isolated in the United Kingdom during 1995 and 4 nontoxigenic strains isolated in other countries were analyzed by PCR and direct sequencing to determine the presence and intactness of thedtxRgenes. The functionality of the DtxR proteins was assayed by testing for the production of siderophore in medium containing high and low concentrations of iron. PCR amplification and sequence analysis of thedtxRgenes revealed four variants of the predicted DtxR protein among the nontoxigenic strains isolated in the United Kingdom. Production of siderophore in medium containing a low concentration of iron and repression of siderophore production in medium containing a high concentration of iron demonstrated that in all the strains thedtxRgenes were functional. These findings demonstrate that, if lysogenised by a bacteriophage, nontoxigenic strains circulating in the United Kingdom could produce toxin and therefore represent a potential reservoir for toxigenicC. diphtheriae.