Mexican authorities have found 123 people from neighbouring countries trapped in a trailer in the central state of San Luis Potosi.
They include 34 children, Mexico's immigration agency said.
Most of those found in Matehuala, 600km (372 miles) north of Mexico City, were from Guatemala, Nicaragua and Honduras.
Officials did not provide details about where the migrants were heading, but many travel through the Americas hoping to reach the United States.
They were found after a local resident reported hearing cries from a locked trailer box, Mexico's National Migration Institute (INM) said, adding that they were being given food and medical assistance.
It also said they were being provided with accommodation until their situation is resolved.
Some of those found were also from El Salvador, Ecuador and Cuba.