Christopher Guest, Michael McKean, Harry Shearer and Rob Reiner will host a private dinner in Los Angeles for whoever wins the bidding war and a guest of their choice, said a news release from Live Earth.
All proceeds will benefit the Live Earth concerts and Alliance for Climate Protection.
The auction for the exclusive dinner was launched Friday and the date will be determined by all the guests' schedules.
"It's an honor to have Spinal Tap reuniting to play at Live Earth and providing this once-in-a-lifetime experience for one of their true fans," said Kevin Wall, founder and producer of Live Earth.
Spinal Tap will perform new material from "Warmer Than Hell," which was written specifically for their July 7 performance at Wembley Stadium during Live Earth London.