One of the two men left critically ill by a drugs trial almost two weeks ago is reported to be making good progress.
The same drug made monkeys' glands swell
Doctors at Northwick Park Hospital, in north-west London, said they were pleased with his progress although he remains in a critical condition.
The other, one of six volunteers who fell ill after taking the TGN1412 drug at a research unit, is seriously ill.
Medical research firm Parexel, which carried out the tests, said its staff followed correct procedures.
A hospital spokesman said: "The two men have made improvements but they remain in critical care.
"One is in a serious condition. The other is still critical but we are pleased with his progress since last week. We remain cautious however because his condition is still complex."
The hospital said the four other men involved in the trial are out of intensive care and are said to be making a good recovery.
"They... are working on regaining their strength and mobility, and are making good progress with both," a hospital statement read.
The case is being investigated by the Medicines and Healthcare products Regulatory Agency (MHRA), which seized documents and sealed off offices.
Scotland Yard said officers were talking to the MHRA and doctors.
TeGenero, which developed the drug, said TGN1412 had caused the glands of two monkeys to swell in earlier tests, but said the symptoms differed from those seen in the men's cases.