We study the mechanics of cartel enforcement and its interaction with bidding constraints in the context of repeated procurement auctions. Under collusion, bidding constraints weaken cartels by limiting the scope for punishment. This yields a test of collusive behavior exploiting the counter-intuitive prediction that introducing min- imum prices can lower the distribution of winning bids. The model’s predictions are borne out in procurement data from Japan, where we find considerable evidence that collusion is weakened by the introduction of minimum prices.