Reeves, Inc. v. Stake, 447 U.S. 429 (1980), was a United States Supreme Court case in which the Court held that individual states, when acting as producers or suppliers rather than as market regulators, may discriminate preferentially against out-of-state residents.

Reeves, a ready mix concrete distribution company in Wyoming, relied on the South Dakota state-owned factory for up to 95% of its cement supplies.

The Supreme Court first promulgated the market participant exception to the negative commerce clause in Hughes v. Alexandria Scrap Corp.,[3] in which Maryland offered a "bounty" for destroying abandoned Maryland automobiles but effectively limited receipt of the bounty to in-state residents.

[3] “Nothing in the purposes animating the Commerce Clause prohibits a State, in the absence of congressional action, from participating in the market and exercising the right to favor its own citizens over others.”[3] Thus, while state laws that prefer intrastate commerce to interstate commerce for economic protectionism are ordinarily invalid per se, states when acting as market participants may engage in such discrimination.

Here, South Dakota was acting as a market participant where Congress had not taken any regulatory action; thus, South Dakota could favor its citizens in the sale of state-produced cement over the citizens of other states.