Very famous poster. The Fillmore Acid Test was the test that the media reported on. It was at this test that the "Warlocks" changed their name to the Grateful Dead.
Mike did this poster at Ken Kesey's ranch during Christmas of '65 while several of the other Pranksters helped with the project.
Only 100 posters were ever printed while the original hung in the Fillmore. Most were stolen as soon as they were placed on the power poles in San Francisco..