The polymerase chain reaction (PCR) was used to discriminate between toxigenic and non-toxigenic isolates of Corynebacterium diphtheriae. Primers specific to the diphtheria toxin gene were used to amplify a toxin gene fragment from simple boiled-cell preparations. Eight recent clinical isolates and four reference strains were tested. The result of the PCR agreed with the traditional toxigenicity assays (the Elek test and guinea pig inoculation) in all cases. PCR has several advantages over the Elek test: it gives a same-day result, it works on colonies taken from selective media, and it detects the toxin gene in mixed cultures. One potential drawback is that the PCR might give a false positive result with the occasional isolate carrying an inactive toxin gene. The good predictive value of a negative PCR result, however, should make it a valuable screening test.