Disturbing new findings have provided a key link in the chain of evidence connecting antibiotics used on livestock to outbreaks of disease caused by antibiotic-resistant human pathogens. Now agencies worldwide are beginning to clamp down on antibiotic use in agriculture. The moves have riled industry officials, who argue that antibiotics are essential to keeping animals healthy and the food supply safe, and that the most serious threat comes from indiscriminate use of antibiotics in people, not livestock.