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 after medication were diluted 5-fold with water, loaded into triplicate 1 L anaerobic digesters and incubated at 35 C. The CTC concentration decreased approximately 75% (front 5.9 to 1.4 ppm) during the 33 days digestion period, yielding it half-life of about 18 days. The concentration of the CTC epimer. 4-epi-chlortctracycline (ECTC). declined roughly 33% (from 4.1 to 2.5 mg/L) during, anaerobic digestion. However, the concentration of the CTC metabolite. iso-chlortetracycline (ICTC), increased 2-fold (from 2.3 to 4.6 mg/L) during the digestion period. Although the water-soluble concentration of CTC decreased 84% (from 0.3 to 0.04 mg/L), the water-soluble concentrations of ECTC and ICTC increased roughly 2-fold during digestion (front 0.5 to 0.93,and 1.0 to 2.7 mg/L. respectively). Since ECTC and ICTC are more water-soluble than the parent tetracycline CTC, it is more likely that these compounds present in digested manure slurry will be detected in water monitoring samples. (C) 2008 Elsevier B.V. All rights reserved.