Scientists appear to be a step closer to transplanting a kidney without the need for a lifetime of drugs.
Even after transplant, a lifetime of drugs awaits a recipient
Two separate US techniques have seen recipients recovering without the need for powerful drugs, which carry many side-effects - including a cancer risk.
Unless the organ comes from an identical twin, the body's reaction is to reject it as a foreign invader.
UK experts said the findings were exciting, but warned a lack of donor organs remained the principal problem.
Close matches require a lifetime of immunosuppressive drugs which increase the risk of infection, high blood pressure and cholesterol.
They can also increase the risks of certain types of cancer.
Doctors have been working for years on ways to stop this process of rejection and reduce the need for drugs.
While most transplant units in the UK have managed to cut the dosage by about 50% in the last five years, the risk of side-effects remains.
The results of both studies appear in the New England Journal of Medicine.
In the first study at Stanford University, a man who received his brother's kidney has gone for two years without drugs after his doctors tweaked his immune system with irradiation and antibody treatments.
He then received an infusion of his brother's blood cells. This combined treatment created a kind of "peacekeeping" immune cell, which appeared able to avert the attack on the foreign organ.
"The idea of getting off drugs holds tremendous appeal for patients," said the study's lead author, Professor John Scandling.
"So far, there is hope, but we still have a long way to go."
Destroying bone marrow
The second study was conducted at Massachusetts General Hospital.
Five patients were given treatment that partially destroyed their bone marrow and with it the white blood cells which cause rejection.
This bone marrow was then replaced with a bone marrow graft from the donor, and the kidney followed.
One of the five rejected the kidney, but the other four have so far been able to live with normal kidney function and without drugs - the longest for over five years.
Dr Robert Higgins, a specialist in renal transplantation, said while both studies were exciting, he had particular concerns about the one carried out in Massachusetts.
"While these early results are encouraging, the treatment involved in destroying the bone marrow can be life threatening, and there are other risks, such as the bone marrow graft trying to 'reject' the patient."
He added: "As in all parts of the world, the main problem in transplantation is the shortage of organs."
In the UK, the prime minister has thrown his weight behind plans to put everyone automatically on the organ donor list, as is done in countries such as Spain, rather than relying on people to sign up.