Peace icon Desmond Tutu checked into a South African hospital on Wednesday for tests related to an ongoing infection, his foundation said.
"Archbishop Emeritus Desmond Tutu has checked into a Cape Town hospital for the treatment of a persistent infection and to undergo tests to discover the underlying cause," the foundation said in a statement.
The treatment did not include any surgery, it said.
A photograph of the 81-year-old Nobel Laureate showed him smiling at his office where he spent the morning, before being admitted to the undisclosed hospital.
"He was in good spirits and full of praise for the care he receives from an exceptional team of doctors," said the statement from the Desmond and Leah Tutu Legacy Foundation.
"The non-surgical treatment is expected to take five days."
Known fondly as "the Arch", Tutu told AFP in an earlier interview that most of his life had "been a bonus".
He survived an illness believed to be polio as a baby, battled tuberculosis as a teenager and prostrate cancer, which he was diagnosed with in 1997.
Nearly 10 years later, he said the cancer had returned after having gone into remission but was non-aggressive.
In December 2011, he underwent minor elective surgery in Cape Town for an undisclosed complaint.
His recent public appearances have shown little hint of ill-health.
Just under two weeks ago he got up to dance at the cathedral where he rallied against the apartheid state as archbishop of Cape Town.
The ruling African National Congress said it had "learnt with concern" of Tutu's hospitalisation.
"We wish him a speedy recovery and trust that he will soon resume his noble duties in the transformative socio-economic agenda of our country," it said.
The main opposition Democratic Alliance said it was "saddened" by the news and wished him "a speedy recovery".
"The archbishop, his friends and family will be in the thoughts and prayers of a great many people around the world tonight," it added.
Officially retired, the outspoken Tutu is still seen as South Africa's moral guide.
He has campaigned on justice and human rights issues around the globe, but has not shied away from pointing out the shortcomings of his own country -- which he christened the "Rainbow Nation."
Earlier this month, at a briefing on his $1.7 million Templeton Prize, he urged South Africa to recover the "spirit that made it great".
"The world was thrilled when freedom came to our land and we pray that South Africa will recover its own sense of worth, we will recover the sense of worth of every single human being," he said.
While his causes might be serious, playfulness is never far from Tutu who is quick to crack jokes -- often directed at himself -- with a trademark uproarious laugh.
A thorn in the apartheid state's side, he campaigned against white minority rule during the years that Nelson Mandela was imprisoned.
He won the 1984 Nobel Peace Prize for his work, which he said has always been motivated by religion.
Tutu is a founding member and chair of The Elders, a group of retired leaders who take on the world's most challenging problems.
His outspokenness has often put him in the firing line.
Just this month, the South African government both praised him for being an inspiration and dismissed his comments that said South Africa was one of the world's most violent nations post-apartheid.
He was ordained at the age of 30 and was appointed the first black archbishop of Cape Town in 1986.
Mandela appointed Tutu to chair South Africa's Truth and Reconciliation Commission, which investigated crimes committed by all sides during apartheid.
He also advised on reconciliation in the wake of conflict, including in Northern Ireland and the Solomon Islands.
He married his wife Leah in 1955 and they had four children.