Escherichia coliisolated from pigs fed on a medicated diet containing 120 p.p.m. halquinol did not develop any resistance to this addition over a 6‐week period.Sensitivity patterns of theE. coliisolates to eight antimicrobial substances, although fluctuating slightly during the test period (but no more than a control group), did not significantly alter. However, the patterns did change significantly when for 17 days after the completion of the halquinol trial the pigs were fed a normal commercial ration medicated with a commonly used feed additive containing chlortetracycline hydrochloride, procaine penicillin and sulphadimidin