Suffolk Ebola patient William Pooley has been treated with an experimental drug, staff treating him at the Royal Free Hospital in north London said.
He was given the first dose of ZMapp on Monday and further doses are expected to be given to him "in due course".
The hospital said Mr Pooley was "sitting up and talking to the nurses and doctors who are looking after him".
Dr Mike Jacobs said: "We are giving him the very best care possible. However, the next few days will be crucial. The disease has a variable course and we will know much more in a week's time.
"Will is in a stable position and we are very pleased with where he is, we couldn't hope for more."
Dr Jacobs said they had acquired the drug through the hospital's "clinical networks" and been supported by "international colleagues".
"What we want to do is give him the best chance we can and that's why we used the ZMapp. He was very, very keen to go ahead and wanted to do so," he said. "We thought there was sufficient reason to offer it to him and have the discussion.
"He considered his options very, very carefully. It is an experimental drug. He wanted to weigh up what we knew about it and he came to the very clear conclusion in his own mind that he would like to go ahead with the treatment."
He said it was "too early" to say what impact the drug has had but added: "Pleasingly, it seems to have had no side effects at all."
Mr Pooley was said to be in "remarkably good spirits given the situation he finds himself in".
"It would be fair to say we couldn't hope to be in a better place today given how far he is into the illness."
Dr Jacobs said public health officials had contingency plans in place for the "unlikely" scenario that another patient needs treatment for Ebola in the UK.
He added it was "business as usual" for the hospital, saying: "We are ready for this and provide for the UK the appropriate facilities."