In the 1949 case Capitol Greyhound Lines et al. v. Brice, Commissioner of Motor Vehicles, the United States Supreme Court addressed a dispute over interstate commerce and state regulation. The appellants were bus companies operating in Maryland who challenged an order by the appellee, Brice as Commissioner of Motor Vehicles for Maryland, which required them to obtain certificates of public convenience and necessity from him before they could transport passengers between points within Maryland on their interstate routes. They argued that this requirement was unconstitutional because it interfered with Congress's power to regulate interstate commerce. The Supreme Court disagreed with the appellants' argument and upheld the validity of state regulations requiring such certificates for intrastate segments of interstate bus routes. The court found that while states cannot interfere with or burden interstate commerce directly, they can impose reasonable conditions on local aspects without violating federal law if those conditions do not conflict with national policy or impede free flow of trade among states.

In the dissenting opinion for Capitol Greyhound Lines et al. v. Brice, Commissioner of Motor Vehicles, 1949 case, it was argued that the majority's decision to uphold Maryland's tax on out-of-state bus companies violated the Commerce Clause of the U.S. Constitution by discriminating against interstate commerce. The dissenters contended that this tax placed an undue burden on interstate businesses and gave preferential treatment to local ones in violation of constitutional principles designed to ensure free trade among states. They also disagreed with the majority’s interpretation of a previous ruling (Spector Motor Service v O'Connor) which they believed should have been applied here as precedent barring such discriminatory taxation practices against interstate commerce.