The feasibility of the anaerobic ammonium oxidation (Anammox) process to treat wastewaters containing antibiotics was studied in this work. Concentrations ranging from 100 to 1000 mg L-1 for tetracycline hydrochloride and from 250 to 1000 mg L-1 for chloramphenicol were tested in batch assays. A strong inhibitory effect was observed for both antibiotics. A concentration of 20 mg L-1 of chloramphenicol was continuously added to an Anammox Sequential Batch Reactor (SBR) system, causing a decrease of the nitrogen removal efficiency of 25%. The Specific Anammox Activity (SAA) of the biomass also decreased from 0.25 to 0.05 g N (g VSS d)-1. Similar effects were observed when 50 mg L-1 of tetracycline hydrochloride were continuously fed. Both antibiotics did not cause any changes in the physical properties of the biomass. A previous degradation step could be necessary in order to treat wastewaters containing inhibitory concentrations of antibiotics by the Anammox process.