A guaranteed lowest prices clause is a commitment that binds the firms that offer it to match their rivals' prices if these are lowest. This paper analyzes the role played by the possibility that some consumers may obtain information on other firms' prices. This information should be required by consumers who are not completely informed. A two stage game is analyzed in which firms simultaneously choose first, whether or not to adopt the guaranteed lowest prices clause and finally, prices. When consumers can complete their information, this game has equilibrium in pure strategies. In this framework the effects of the use of the commitment on social welfare are negative and would not emerge in a complete information setting. These effects are due to the interest that firms have in not covering the market, and in ignoring those consumers whose initial information is incomplete.

Keywords: guaranteed lowest prices, collusion, restrictive practices on competition.