Four men left seriously ill after taking part in a drug trial have regained consciousness, say doctors.
The six are being treated at Northwick Park hospital
Another two are in a critical condition and under sedation but have shown early signs of responding to treatment, said Dr Ganesh Suntharalingam.
The men suffered multiple organ failure within hours of taking the TGN1412 drug at a research unit based at Northwick Park Hospital in north-west London.
Medical research firm Parexel said its staff followed correct procedures.
Herman Scholtz, head of Parexel, praised staff's swift response when volunteers suffered a reaction during tests of an anti-inflammatory drug.
In a statement, posted on the website of US-based Parexel, Dr Scholtz said: "An initial review at the site has shown that best practices were followed.
"We remain in close and constant contact with the Northwick Park Hospital concerning the medical status of the six volunteers."
Dr Suntharalingam, head of intensive care, said: "Some of them have made noticeable progress in response to our treatment and we have been able to reduce the amount of organ support that is required.
"However, it is early days and they will clearly still continue to need specialist observation for some considerable time.
"There are also some very early signs of response to treatment in the most critically ill patients but I must stress that their condition remains very serious and complex and it wouldn't be sensible to comment on prognosis."
On Friday, Labour MP for Brent North, Barry Gardiner, visited the North West London Hospital Trust chief executive Mary Wells, and said there was no better place for the patients to be treated.
He added it was unclear how long the inquiry by the UK medicines watchdog would take.
The Medicines and Healthcare products Regulatory Agency (MHRA) is investigating whether the reaction suffered by the men was caused by a manufacturing problem, contamination, a dosing error or whether it was some "completely unanticipated side-effect of the drug in humans".
Mr Gardiner said: "MHRA has sealed the offices of Parexel, bagged and taken away all the relevant information, is conducting interviews with Parexel and with their staff."
Speaking to the BBC on Thursday, Lord Winston, a fertility expert, rejected claims regulation of clinical testing of new drugs was "weak and ramshackle".
The professor of fertility studies at Imperial College London and vice-chairman of the Parliamentary Office for Science and Technology, said Britain's system of clinical trials had "lots of safeguards".
He was responding to concerns raised earlier by Paul Flynn MP.
"I think it's really unfortunate that there might be given an impression that our very ethical drug industry is actually not working according to proper practice because I think on the whole it undoubtedly is," he said.
He said he was concerned about growing resistance to animal testing.
He said: "I wonder really whether in fact there's increasing reluctance to do the preliminary trials on animals because of the difficulties generally in doing animal research.
"That I think is a disaster for humans."
'Tested on monkeys'
TeGenero, which manufactures the drug, apologised to the sick men's families and said the medicine had shown no signs of problems in earlier tests.
The company's chief scientific officer, Thomas Hanke, said he and his colleagues were "devastated" by what had happened.
He said TGN1412 had been tested extensively in laboratories and on rabbits and monkeys, with no adverse effects and no drug-related deaths.
It was the first time the drug, designed to treat conditions such as rheumatoid arthritis, leukaemia and multiple sclerosis, had been tested on humans.
Scotland Yard said officers were talking to the MHRA and doctors.