The court held that a municipal ordinance requiring all milk sold in Madison to be pasteurized at an approved plant within 5 miles of the city, unconstitutionally discriminated against interstate commerce.
Illinois milk producer, Dean Milk, on appeal from a state court holding that found the municipal ordinance to be reasonable, charged that the true purpose of the ordinance was to protect local industries from competition from non-local producers.
In the court's opinion, Justice Clark said: "In thus erecting an economic barrier protecting a major local industry against competition from without the state, Madison plainly discriminates against interstate commerce. This it cannot do, even in the exercise of its unquestioned power to protect the health and safety of the people, if reasonable nondiscriminatory alternatives... are available".
The fact that in-state producers were also discriminated against was not found to be relevant to the fact that it discriminated against interstate commerce.