The aim of this study was to investigate the effect of a common veterinary antibiotic in biogas plants. 20 mg/kg of oxytetracycline was intramuscularly injected into a cow and its concentration in manure, which was sampled daily during the following 20 days, was measured. A total of 20 % of the injected oxytetracycline was detected in manure. Collected manure samples on days 1, 2, 3, 5, 10, 15, and 20 were digested in triplicate serum bottles at 37 A degrees C for 30 days. Control serum bottles produced 255 +/- A 13 mL biogas, whereas 50-60 % inhibitions were obtained for the serum bottles operated with samples collected for the 5 days after medication. Multivariate statistics used for the evaluation of FISH results showed that Methanomicrobiales were the main methanogenic group responsible for most of the biogas production. Numbers of active Bacteria and Methanomicrobiales were negatively correlated with the presence of oxytetracycline, whereas Methanosarcinales and Methanobacteriales were less affected.