To explore the response of antibiotic resistance genes (ARG) to biological treatment, dairy lagoon water was incubated anaerobically or aerobically at 20 degrees C or 4 degrees C. Three conditions were compared: Antibiotic (Ab) Spiked, Ab Spiked and Killed, and Background (unamended). For Ab Spiked conditions, oxytetracycline, sulfamethoxazole, tylosin, and monensin were each added at 20 mg/L. Antibiotics and ARG were monitored using high-performance liquid chromatography/tandem mass spectrometry and quantitative real-time polymerase chain reaction, respectively. Biological degradation of antibiotics in all treatments and varied responses of different ARG was observed. Aerobic versus anaerobic treatment had no effect on tet(W), with an overall pattern of increase in the presence of antibiotics followed by decrease to initial levels. tet(O) responded differently under aerobic versus anaerobic treatment, increasing to highest levels at 4 degrees C under aerobic treatment and at 20 degrees C under anaerobic treatment before returning to initial levels. sul(I) and sul (II) showed similar patterns and increased in all Ab Spiked conditions, failing to return to initial levels at 4 degrees C and in some of the 20 degrees C treatments. ere(A) and msr(A) were lower than the other two ARG classes and remained constant in all treatments.