An emergency operation for Lisa, life-long mate of Max the gorilla, was planned for Wednesday to remove a possible cancerous growth, the Johannesburg Zoo said.
A few weeks ago, a keeper noticed a lump next to Lisa's breast which veterinarians suspected may be cancer, the zoo said in a statement on Tuesday.
Due to the worsening condition of the lump, the zoo's vets decided to schedule an emergency operation for Lisa for Wednesday, to be performed by senior veterinarian Michelle Barrows assisted by two top Gauteng human specialist doctors.
Results of the operation were expected next week.
The 35-year-old Lisa was the mate of Max the gorilla, who came to the public's attention after helping to foil an armed robber who jumped into the primates' enclosure in the late nineties.
Max attacked the armed man who jumped into the enclosure while he was being pursued by police. Max is said to have interpreted the man's presence as a threat to Lisa.
The man shot Max twice and both the attacker and gorilla were hospitalised after the encounter.
Max died of natural causes aged 33 in May 2004.