Last year’s landmark Wal-Mart case could help two other companies facing class action lawsuits in the Supreme Court today.

In Dukes v. Wal-Mart Stores, the high court on June 20, 2011, rejected class certification for a lawsuit in which plaintiffs sought to represent more than 1.5 million women Wal-Mart employees who claimed the retailer discriminated against them in pay and promotions. The court said the members of the proposed class held different jobs nationwide with different supervisors and thus didn’t have enough in common to constitute a single class.

Experts say Dukes is likely to help Comcast Corp. and Amgen Inc., which both face class actions that are being argued before the Supreme Court today. Stanford Law School Professor Deborah Hensler told Bloomberg Businessweek that Dukes “is making it more and more difficult to certify class actions” and that plaintiffs “need to prove a lot about the merits at the certification stage.”

Seyfarth Shaw Partner Gerald Maatman Jr. told Bloomberg Businessweek that between June 2011 and Dec. 31, 2011, there were 260 rulings in state and federal court that applied Dukes. Since then, at least 800 more rulings have cited Dukes.

Comcast faces an antitrust lawsuit that seeks $875 million on behalf of more than 2 million subscribers and former subscribers to the cable company’s services. Comcast claims a federal judge improperly certified the class before resolving whether the plaintiffs could prove damages as a group.

Amgen faces a securities fraud class action suit in which plaintiffs claim the company and its executives misled investors about safety questions involving two of its drugs. The Supreme Court will consider whether the plaintiffs in the case must prove that allegedly misleading information affected the company’s stock price before being certified.

Read more InsideCounsel stories about class actions and class certification:

Temps file class action against Wal-Mart, workers threaten walkout

Starbucks appeals $14.1 million tip-pooling class action suit

Marketplace realities get lost in class certification “shortcut”

Supreme Court will consider reach of Class Action Fairness Act

Federal courts scrutinize class certification petitions