Nola, the lone northern white rhino at the San Diego Zoo Safari Park, is receiving antibiotics after a large swelling was lanced on her right hip.
Pathologists are examining liquid and tissue samples from the basketball-sized abscess for any signs of illness or disease, including cancer, officials said.
The 41-year-old rhino, one of only five northern white rhinos in the world, has no lack of energy and appetite and remains on exhibit.
In December, Nola was treated for a possible sinus infection after keepers noticed that her appetite and energy level were down.
Nola took well to the lancing and now the daily cleansing of the abscess with sterile saline. "Nola is a great patient," said Meredith Clancy, associate veterinarian at the park.
Poaching in Africa has driven the northern white rhino to the brink of extinction. Three are in the Ol Pejeta Conservancy in Kenya and one in the Dvur Kralove Zoo in the Czech Republic.
The Safari Park's other northern white rhino, the male Angalifu, died in December at age 44.
On Friday, the park is holding an Endangered Species Day event, including a "Rally 4 Rhinos."