For many decades, antibiotics have been used in large quantities in the poultry, beef, and swine production industries to promote animal growth. Of the nearly 25 million pounds of antibiotics used by the US livestock industry, 25 to 75% is excreted in the manure and applied to agricultural lands. Very little is known about antibiotic effects on N cycling in soils, although such affects could have a major impact on both crop productivity and water quality in states like Kentucky where livestock antibiotics are widely used and distributed.  The main objective of this study was to determine the effect of nine antibiotics commonly used in the poultry industry on ammonification, nitrification, and denitrification by soil microorganisms. Antibiotic effects on aerobic and anaerobic N cycling processes were evaluated in the laboratory using soil microorganisms extracted from soil with a weak saline solution. Ammonification under aerobic conditions was inhibited by penicillin, virginiamycin, and oleandomycin at ³ 3 ppm, but not by other antibiotics, while ammonification under anaerobic conditions was inhibited by all antibiotics between 0.1 to 30 ppm. Nitrification was inhibited by virginiamycin and oleandomycin at ³ 10 ppm, but not by other antibiotics. Denitrification was inhibited by five antibiotics (bacitracin, chlortetracycline, erythromycin, flavomycin, and virginiamycin) at between 0.1 and 30 ppm, but not by other antibiotics. This study showed for the first time that all FDA-approved antibiotics used in the poultry industry had inhibitory effects on at least one N cycling reaction conducted by soil microorganisms.