Reverse-transcription PCR (RT-PCR) was used to assess the effects, in vitro, of rifampicin on the transcription of the eaeA, stx1 and stx2 genes (coding for intimin and shiga-like toxins I and II, respectively) of seven strains of Escherichia coli O157:H7 associated with human infection. Each strain was found to possess all three genes and, in the absence of rifampicin, all seven strains transcribed eaeA and stx2 (three strains did not transcribe their stx1 genes). Transcription of all three genes was inhibited (as witnessed by a negative result in the RT-PCR), however, when the strains were incubated, at 37 degrees C, with rifampicin at 4 microg/ml (found to be the minimum concentration of this antimicrobial agent that inhibited the multiplication of E. coli O157:H7). The results of an assay based on reversed passive latex agglutination (RPLA) revealed that exposure of the bacteria to 4 microg rifampicin/ml led to a 12-fold decrease in the release of shiga-like toxin I and a 16-fold decrease in the release of shiga-like toxin II. As rifampicin is capable of inhibiting the in-vitro transcription of the genes encoding the shiga-like toxins and intimin attachment protein of E. coli O157:H7, it may be useful in the treatment of human infections with strains of this bacterium. Studies are now underway to assess the in-vitro and in-vivo effects of rifampicin, at both transcription-inhibitory and bactericidal concentrations, on E. coli O157:H7. The effects of other agents on the inhibition of the expression or activity of the shiga-like toxins and intimin attachment protein will also be determined.