An orange light bulb.
The light bulb was “shoved in the case on-set” at the last-minute, creating the glow you see every time it opens.
You’re probably looking for a more ingenious answer than that – because what interests fans is what was in the briefcase in the story, rather than what was in there on set.
Sadly there is no answer.
In Roger Ebert’s book Questions for the Movie Answer Man, Pulp Fiction co-author Roger Avery explains that “no prop master can come up with something better than an individual’s imagination”.
Thus the suitcase contains whatever the individual film-watcher imagines it does. And boy do people have some weird imaginations.
Dazed Digital ran a series of the wackiest fan theories, which include Elvis’ gold suit, a small nuke and the soul of Marsellus Wallace.
As for us, we’re pretty sure it has an Oscar statue in it to make up for the fact Forest Gump beat Pulp Fiction to the little gold statue that year at the Academy Awards.
There really is no justice.