Actor Alan Cumming is offering a $10,000 (£8,000) reward for information about the whereabouts of a chimpanzee who starred in the 1997 comedy, Buddy, with him.
Tonka was last seen in a cage at a now-defunct Missouri Primate Foundation, a chimpanzee breeding facility that rented the primates out for movies and parties or sold them to private owners.
Animal rights group PETA said Cumming is doubling its reward, raising it as high as $20,000 (£16,000) for information about the chimpanzee.
Missouri Primate Foundation has come under repeated criticism from animal rights groups who claim the company mistreated its chimpanzees.
The facility has been closed and no one seems to be able to locate Tonka.
Cummings said: “During the months we filmed together, baby Tonka and I became good friends, playing and grooming each other and just generally larking about.
“It’s horrible to think he might be in a cage in a dark basement somewhere or have met some other fate, so I’m appealing to whoever knows what has become of him to please come forward and claim the reward.”
The actor has previously raised concerns about Tonka’s conditions in the Missouri facility and has campaigned alongside animal rights groups.
Sky News tried to contact people connected with the now defunct business.
PETA previously sued Missouri Primate Foundation over the living conditions of the chimpanzees there and was granted permission last year to rescue Tonka along with six others and move them to a sanctuary.
However last July, when PETA came to carry out the rescue, Tonka was not there.
PETA’s Brittany Peet said: “If you know where Cumming’s former co-star may have been shipped to, sold, or hidden away, PETA wants to hear from you.
“If he’s still alive, Tonka deserves to live out the rest of his days surrounded by chimpanzee friends at a lush sanctuary, as ordered by the court, and someone out there might be able to help PETA get him there.”
Buddy featured Cumming, alongside Robbie Coltrane and Rene Russo, and was about a couple who adopt and raise a feeble young gorilla.