This is a test. This is only a test. President George W. Bush's Cabinet is testing on Saturday the government's readiness to respond if several improvised explosive devices were to be detonated across the nation. Members of the Cabinet and dozens of other officials from the Justice Department, FBI, Pentagon, Homeland Security Department and other agencies will conduct the drill in the Eisenhower Executive Office Building next to the White House. Bush will not participate. The exercise, led by the president's Homeland Security adviser, Fran Fragos Townsend, is not based on any threats to the United States, White House deputy press secretary Scott Stanzel said Friday. "There is a fictional scenario and the officials talk about how they would react in the event of this fictional disaster," he said. It will be the fourth in a series of drills since December 2005 intended to simulate the Cabinet's role, responsibilities and authority in responding to a disaster. Others dealt with pandemic flu, a possible smallpox outbreak and a catastrophic hurricane.