Using data on wholesale prices for antibiotics sold to U.S. drugstores, we test the growing theoretical literature on ‘countervailing power’ (a term for the ability of large buyers to extract discounts from suppliers). Large drugstores receive a modest discount for antibiotics produced by competing suppliers but no discount for antibiotics produced by monopolists. These findings support theories suggesting that supplier competition is a prerequisite for countervailing power. As further evidence for the importance of supplier competition, we find that hospitals receive substantial discounts relative to drugstores, attributed to hospitals' greater ability to induce supplier competition through restrictive formularies.