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This paper examines the output and profit effects of horizontal mergers between up-stream firms in intermediate-goods markets. We consider market settings in which the upstream firms sell differentiated products to a downstream retail monopolist. We find that if the merging firms can bundle their products, transfer pricing is efficient before and after the merger. The merging firms gain and the retailer loses, but absent any cost efficiencies, consumer and total welfare do not change. If the merging firms cannot bundle their products, the effects of the merger depend on the merged firm’s bargaining power. If the merged firm’s bargaining power is low, the welfare effects are the same as in the case with bundling; if its bargaining power is high, and there are no offsetting cost efficiencies, the merger typically reduces welfare. We evaluate the profit effects of merg-ers on the rival firms and retailer for the case of two-part tariff contracts. Contrary to conventional wisdom, we find that a merger that harms the retailer may increase welfare