The Federal Trade Commission sued PepsiCo on Friday, alleging that it has engaged in illegal price discrimination by giving unfair price advantages to one large retailer at the expense of other vendors and consumers.

The US Federal Trade Commission sued PepsiCo Inc. Friday under a rarely invoked 1930s law called the Robinson-Patman Act that bars price discrimination against retailers.

According to the FTC’s complaint, Pepsi has been providing unfair pricing advantages to one of its largest customers—a major big box retailer—while raising prices for competing retailers and customers. Reuters reports that the retailer is none other than Walmart Inc. WMT , citing a source familiar with the discussions.