A plaintiff brought a federal claim against defendants who invoked a state statute purporting to immunize them from liability. The Supreme Court held that a State cannot extinguish or condition a cause of action created by federal law. Under the Supremacy Clause, state immunity rules must yield when they conflict with the substantive liability Congress imposed. States retain authority over their own causes of action and neutral procedural rules, but they may not confer a defense that defeats a federal right because of disagreement with federal policy. The judgment was reversed and the case remanded.