According to Reuters, the Antitrust Division has opened an investigation into the proposed revenue sharing agreement between Yahoo and Google. Under the agreement, Yahoo will allow Google to put advertisements on its site in exchange for a share of the revenue. Google and Yahoo are reported to have shares of about 80% and 16% respectively of online advertising revenue. The obvious concern is whether the agreement will reduce the incentives for Google and Yahoo to compete and therefore, violate Section 1 of the Sherman Act. Yahoo may have an incentive to raise its prices knowing that under the agreement, it will share in any lost business to Google. The Antitrust Division reportedly has issued civil investigative demands not just to Google and Yahoo but to many other players in the industry. Although not required to do so, Google and Yahoo agreed not to go forward with their collaboration until the Antitrust Division has an opportunity to review the potential effects on competition. The parties have attempted to shrug-off the investigation as expected. But it certainly is not routine. The Antitrust Division does not take issuance of CIDs lightly.