A set of four synthetic oligonucleotide probes derived from sequences of the VT1 (Shiga-like toxin I [SLT-I]) and VT2 (SLT-II) genes were used in a polymerase chain reaction (PCR) amplification procedure to detect these genes in some enteric pathogens. A total of 40 verotoxin-producing Escherichia coli strains and 43 isolates of other recognized enteric pathogens were studied. PCR amplification products identifying the VT1 and VT2 gene sequences were observed only in nucleic acid extracted from strains found to be VT positive in traditional tissue culture assays. Template nucleic acid extracted from other gram-negative bacteria was found to be negative with the exception of five isolates of Shigella dysenteriae type 1 in which good amplification with the VT1 probe was observed. The oligonucleotide probes clearly distinguished VT1 and VT2 strains of E. coli and did not give specific amplification with nucleic acid from VTe (a SLT-II variant)-producing E. coli. VT1 or VT2 genes or both were not detected in E. coli K-12 strain C600 or HB101 or in strains known to express other virulence factors, such as enterotoxins, adhesins, hemolysins, or unrelated cytotoxins. The sensitivity of the PCR procedure for detection of both VT1 and VT2 genes was determined to be 1 ng of total nucleic acid. Furthermore, the VT1 gene was easily detected when only 100 pg of nucleic acid was used as the template in the PCR procedure.