(1918), legal case in which the Supreme Court of the United States struck down the Keating-Owen Act, which had regulated child labour. The act, passed in 1916, had prohibited the interstate shipment of goods produced in factories or mines in which children under age 14 were employed or adolescents between ages 14 and 16 worked more than an eight-hour day.

Hammer v. Dagenhart was a test case in 1918 brought by employers outraged at this regulation of their employment practices. Dagenhart was the father of two boys who would have lost jobs at a Charlotte, N.C., mill if Keating-Owen were upheld; Hammer was the U.S. attorney in Charlotte.

In a 5 to 4 decision, the Court ruled that the Keating-Owen Act exceeded federal authority and represented an unwarranted encroachment on state powers to determine local labour conditions. In a notable dissent, Justice Oliver Wendell Holmes (Holmes, Oliver Wendell, Jr.) pointed to the evils of excessive child labour, to the inability of states to regulate child labour, and to the unqualified right of Congress to regulate interstate commerce—including the right to prohibit.

Hammer v. Dagenhart was overturned when the Court upheld the constitutionality of the Fair Labor Standards Act in U.S. v. Darby Lumber Company (1941).