Southern hybridizations of potential ETEC strains isolated from ostriches. HindIII-digested genomic DNAs isolated from four ETEC ostrich isolates (OS-1, OS-2, OS-3, and OS-4), E. coli SA53 (LT-IIa), E. coli Ec41 (LT-IIb), and E. coli DH5α (control strain) were hybridized with nucleotide probes homologous to genes encoding LT-IIa holotoxin (A), LT-IIb holotoxin (B), B polypeptide of LT-IIa (C), or B polypeptide of LT-IIb (D). The gene encoding the A polypeptide of LT-IIa contains a HindIII restriction site. Thus, digestion of SA53 genomic DNA with HindIII produced two hybridizing fragments. Southern hybridization conditions were chosen for moderately high stringency to detect DNA fragments having closely related nucleotide sequences.