The Slaughterhouse Cases is the collective name given to matters brought by Louisiana meat packing companies and heard by the U.S. Supreme Court in the late 19th century. It is historically significant because it gave the court its first shot at interpreting the Fourteenth Amendment, and because of the court's ruling on the parameters of the amendment's Privileges and Immunities Clause.
In its ruling on the Slaughterhouse Cases, the Supreme Court took a controversial view regarding the Privileges and Immunities Clause of the Fourteenth Amendment. Specifically, the court determined that the basis for protection afforded by the clause is U.S. citizenship, not state citizenship. It then applied a narrow definition to U.S. citizens' rights that did not include civil rights.
The dissenting justices and others who have disagreed with the decision over the years said it basically nullified the clause altogether.