Starting Tuesday, air travelers also will be able to buy drinks or other liquids or gels at shops inside airport security checkpoints and carry them on board under partially relaxed anti-terror rules.

If a passenger brings a container larger than 3 ounces from outside, it will still have to be put in checked baggage.

The outright ban on such carry-on items, ordered Aug. 10 after an alleged plot to bomb U.S.-bound jetliners was foiled, is no longer needed, Transportation Security Administration chief Kip Hawley said Monday.

The FBI and other laboratories tested a variety of explosives and found that tiny amounts of substances, so small they fit into a quart-size plastic bag can't blow up an airliner, Hawley said at a news conference at Reagan National Airport.

Lisa Cohen, a congressional staff member who flies weekly between Denver and Washington, said the changes should make flying easier.

"I understand the concerns, but it's been a colossal pain this last several months," Cohen said as she lugged a large carry-on bag through the airport.

At Hartsfield Jackson Atlanta International Airport, Aubrey Hux wasn't as happy to hear the news. Just back from a vacation in Thailand, he said, "You can't have enough security as far as I'm concerned."

Doug Zink, who traveled to Washington from Denver on business, doubted whether the average passenger would remember that containers have to be 3 ounces or less, that plastic bags have to be one quart or less and that the bags have to have a zip top that's closed.

"It's just ridiculous," Zink said.

Up to 4 ounces of a few items will be permitted in carry-on bags: eye drops, saline solution, nonprescription medicine and personal lubricants.

Larger bottles of liquids and gels from outside, including shampoo, suntan lotion, creams and toothpaste are allowed only in checked baggage.

The Air Transport Association, which represents major airlines, said the TSA has carefully assessed which items can be brought aboard safely.

"It will reduce passenger inconvenience," said ATA President James May.

Passengers will have to take out the clear bags with toiletries in them so they can be checked separately by the X-ray machine. Though the machines can't identify whether a substance is an explosive, they can pick out anomalies that may indicate a substance is intended for use in a bomb.

The TSA is testing new equipment that can detect explosive substances at checkpoints, Hawley said. He said he hopes machines that use magnetic resonance imaging (MRI) technology can be deployed at the nation's 753 checkpoints located in 452 airports.

"A year from now you will know a whole lot more," Hawley said, declining to give a timeline.

He said the TSA has money to pay for such equipment.

However, Massachusetts Rep. Ed Markey, a senior Democrat on the House Homeland Security Committee, criticized the Bush administration for failing to make progress on technology to detect explosives, though a plot to blow up airliners was uncovered 10 years ago.

A Homeland Security spending bill expected to pass this week will provide only a fraction of the money needed for bomb-detection equipment, Markey said.

"Passengers need more than just press conferences, but real progress to upgrade explosive detection equipment at our airports," he said.

Tougher airport screening procedures were put in place in August after British police said they broke up a terrorist plot to assemble and detonate bombs using liquid explosives on airliners crossing the Atlantic Ocean from Britain to the U.S.

At the time, the Homeland Security Department briefly raised the threat level to "red," the highest level, for flights bound to the United States from Britain. All other flights were at "orange" and will remain at orange, the second-highest level, for now.

"Obviously, there's been a lot of unhappiness," said Richard Marchi, senior adviser to the Airports Council International, an airport trade group. "They're right to find a way to ease the burden and maintain a reasonable level of security."