The nucleotide sequence of the LT-BH cistron (eltBH) from an enterotoxigenic Escherichia coli strain infectious for humans was determined and compared with the LT-B cistron sequence from a porcine E. coli isolate. Both cistrons were shown to comprise 375 nucleotide base pairs, and discrepancies were detected at eight positions. Of the nonhomologous base pairs, six resulted in codon changes that would lead to amino acid variations. The nucleotide sequence distal to both LT-B cistrons was also determined, and only three differences were detected in 197 base pairs. An HhaI site unique to eltBH was shown to be present in all the heat-labile (LT) genes from 31 human isolates surveyed, whereas the restriction enzyme recognition site was absent in the gene from 46 porcine E. coli isolates. The results suggest that two genetically discernable LT groups are identifiable and that the groups are also distinguishable by the isolation source (human or porcine) of the infecting E. coli strains.