A startled security worker noticed the shape of a child on the carry-on baggage screening monitor and immediately pulled him out, the Los Angeles Times reported for a story in Wednesday's editions.
The infant was taken to a local hospital, where doctors determined he did not receive a dangerous dose of radiation.
"This was an innocent mistake by an obviously inexperienced traveler," said Paul Haney, deputy executive director of airports and security for the city's airport agency.
The incident happened early Saturday, airport officials said.
Haney said in 1988, an infant in a car seat went through an X-ray machine at LAX.
"Since then LAX has served more than 1 billion travelers without an incident of this type," he said.
Nico Melendez, a spokesman for the Transportation Security Administration, which manages LAX screeners, said the agency doesn't have enough workers to constantly stand at tables in front of the screeners to coach travelers. Still, he said the agency works hard to educate passengers.
The agency's Web site posts tips for travelers, including a section on traveling with children. There, it says, "Never leave babies in an infant carrier while it goes through an X-ray machine."
At the airport, signs also are posted in Spanish and English at ticket counters and near security checkpoints telling passengers to put cell phones, pagers, car keys and other metal objects into bins that go through X-ray machines.