The zoo also took tissue samples from the gorilla, a female named Colo.
The oldest known gorilla living in a zoo is doing well after a surgical biopsy ahead of her 60th birthday on December 22.
The Columbus Zoo and Aquarium said on Saturday that veterinarians successfully removed a mass under the gorilla's arm that recently started causing her discomfort.
The zoo also took tissue samples from the gorilla, a female named Colo, to determine the cause and possible treatment. She also got a thorough medical check-up.
Colo will remain off-exhibit while she is observed after the surgery.
Colo was the first gorilla born in a zoo. She has surpassed the usual life expectancy by two decades.
Colo is a mother of three, grandmother of 16, great-grandmother of 12 and great-great-grandmother of three.