After testing determined there was nothing dangerous about the substance, the 77 passengers were put back on the plane and it made its flight to Jacksonville.
"It was a false alarm, but it was to be on the safe side," said Southwest spokesman Paul Flaningan.
He said Flight 602, scheduled to take off from Birmingham, Ala., at 6:20 p.m., was delayed when a flight attendant noticed a white powdery substance coming from a box in an overhead bin.
Authorities decided to remove passengers from the plane and a hazardous materials team went on board to check it out.
A passenger said he thought the white substance was foot powder he had packed, and testing showed that was the case, Flaningan said.