We empirically examine the impact of relationships between contractors and subcontractors on firm pricing and entry decisions in the California highway procurement market using data from auctions conducted by the California Department of Transportation. Relationships in this market are valuable if they mitigate potential hold-up problems and incentives for ex post rene- gotiation due to contractual incompleteness. An important characteristic of informal contracts are that they must be self-enforcing, so that the value of relationships between ¯rms and suppliers depend on the extent of possibilities for future interaction. We construct measures of the stock of contractors' prior interactions with relevant subcontractors and ¯nd that a larger stock of relationships leads to lower bids and a greater likelihood of entry. Importantly, this relationship does not hold in periods of time and areas with little future contract volume, suggesting that the self-enforcement mechanism is crucial in providing value for informal contracts.