Ds manufacture and distribute concrete block in the Greater Chicago area. They sell to masonry contractors, who submit bids to general contractors for the masonry portions of construction projects. The general contractors, in turn, submit bids for these projects to customers such as Ps. Ps are indirect purchasers of concrete block, which passes through two separate levels in the chain of distribution before reaching Ps. Ps brought this antitrust treble damages action under § 4 of the Clayton Act, alleging that Ds had engaged in a combination and conspiracy to fix the prices of concrete block in violation of § 1 of the Sherman Act. Ps alleged that the amounts paid for concrete block were more than $3 million higher by reason of this price-fixing conspiracy. Ds moved for partial summary judgment in that Ps were indirect purchasers of concrete block and that as a matter of law only direct purchasers could sue for the alleged overcharge. The District Court agreed, but the Court of Appeals reversed, holding that indirect purchasers such as Ps, in this case, can recover treble damages for an illegal overcharge if they can prove that the overcharge was passed on to them through intervening links in the distribution chain. Ds appealed, and the Supreme Court granted certiorari.