Suppliers face increasing pressure from their customers to improve their environmental performance. When firms downstream in the supply chain seek to achieve such improvements themselves, they frequently request that their suppliers adopt greener practices. This paper investigates the rationale for suppliers to comply with or resist the mandate of their customers to adopt the international environmental management standard ISO 14001 in the North American automotive industry. We argue that the effectiveness of such a mandate will vary according to the characteristics of the relationship between suppliers and customers. We contrast and test hypotheses based on both transaction cost and information theories to suggest that suppliers, whether in a dependent or distant relationship with their customers, have incentives to comply with the requests of their customers but through different mechanisms. Our study analyzed the characteristics of 3,152 automotive suppliers located in the United States, Canada, and Mexico over the 2000–2003 period. Findings indicate that suppliers with highly specialized assets, as well as younger suppliers, suppliers headquartered in Japan, and those reporting to the Toxic Release Inventory, are more likely to respond to their customers' pressures to adopt the certified management standard ISO 14001.