Forty-three isolates of Escherichia coli, Salmonella dublin and S. thyphimurium of animal origin were studied by colony hybridization using an intragenic probe specific for the gene encoding 3-aminoglycoside acetyltransferase type IV, which confers resistance to apramycin and gentamicin. Non-radioactive biotinylated and sulphonated DNA probes were used. No false-positive results were found and the positive results obtained were in agreement with those previously revealed with radioactive probes.