Soy milk maker White Wave has filed suit against Suiza Foods Corporation and Dean Foods Company over their proposed merger.

The conflict arises because a subsidiary of Dean Foods currently owns 36 percent of White Wave, and because Suiza is White Wave’s largest competitor in the soy milk business. If the $1.5 billion merger goes through as proposed, Dean’s ownership stake in White Wave would transfer to Suiza.

The suit lists two main points. One, the merger would illegally give Suiza a stake in White Wave. And two, that Suiza and Dean are refusing to honor White Wave’s agreement with Dean that “prohibits any shareholder from transferring shares without giving White Wave and it’s shareholders first chance to purchase the shares.”

Dean and Suiza plan to fight the lawsuit.

The lawsuit is the latest potential obstacle to the proposed deal to unite Suiza, the largest U.S. dairy processor and distributor, with Dean, it’s largest rival.