The fate of antibiotic residues in the manure of treated animals is of considerable concern because of the potential development of antibiotic-resistant bacteria in the environment. The objective of this study was to determine the fate of chlortetracycline (CTC) during the anaerobic digestion of manure from medicated calves. Five beef calves were medicated for 5 days with 22 mg/kg/day of CTC. Manure samples collected from calves prior to and after medication were diluted 5-fold with water, loaded into triplicate 1 1 anaerobic digesters and incubated at 35 degrees C. Approximately 75% removal of buffer extracted CTC was achieved in 33 days by anaerobic digestion, yielding a half-life of about 18 days. Although the levels of buffer extracted CTC epimer, 4-epi-chlortetracycline (ECTC), declined during anaerobic digestion, buffer extracted and water soluble CTC metabolite, iso-chlortetracycline (ICTC), concentrations increased. In addition, water soluble ECTC levels also increased during anaerobic digestion. Because degradation products distributed more towards the water phase compared with the parent CTC, the degradation products present in digested manure slurry will be the ones potentially encountered in water.