Antibiotics and other antibacterials are often applied to the drinking water of food animals. This type of medicationis not always justified and allowed. Monitoring of drinking water for the presence of antibiotics may be indicatedin many cases. Microbiological growth inhibition tests are cheap and simple methods intended to detect antibacterialactivity in different matrixes. Therefore, a method intended for the detection of antibiotics in kidneys was modifiedand optimized for analyzing water.The detection limits of most antibiotics used in animal husbandry were much lower than the therapeutic concentrationsapplied in drinking water. However, with sulphadimidine, lincomycin and spectinomycin, the range wasrelatively narrow. Zinc bacitracin was not detected at the level used in the prevention of mucoid enteropathy of rabbits.Tests in a pig farm revealed high variability in the levels of amoxicillin and doxycycline in samples taken fromdifferent drinking nipples. This finding shows that not all animals were treated in the same way, and that they probablydid not receive identical doses. Neither amoxicillin, nor doxycycline was detectable three hours after treatment.Acidification of drinking water with organic acids produced small inhibition zones. Bacterial growth inhibitionin drinking water is not always due to the presence of an antibiotic, because false positive results may occur. Althoughthe method investigated is not quantitative, it is concluded that it may be useful in the monitoring of drinking watermedication in animal farms.