This comes as doctors said the condition of four of the six men is continuing to improve after almost a week in intensive care.
The German company that developed the TGN1412 drug has claimed the monkeys' symptoms were completely different from those suffered by the men, who are being treated at Northwick Park Hospital in north-west London.
The company TeGenero, insisted that it followed correct procedures during the trial of the drug.
It said that two out of 20 monkeys used in pre-clinical tests had experienced a "transient increase in size of lymph nodes," however the men suffered completely different symptoms.
The company said in a statement that information on the chances of temporary swelling was given to UK regulators ahead of the trial, and was also included on the consent form signed by volunteers.
"We are encouraged by the progress being made by the volunteers, but remain deeply concerned for all of them as they continue their treatment," TeGenero chief scientific officer Thomas Hanke said.
"We are continuing to do all we can to ensure that the investigations into what went wrong proceed as quickly as possible."
Four men stabilising
Three of the four men whose conditions have improved had now been removed from organ support, Dr Ganesh Suntharalingam, clinical director of intensive care at the hospital, said on Sunday evening.
But two other patients remained in a critical condition and despite some early signs of improvement, it was still too early to comment on their prognosis, he added.
All six patients who fell ill last Monday after being injected with the drug are continuing to receive specialist anti-inflammatory treatment.
The previously healthy men had volunteered to test the medication, developed by TeGenero, which was designed to treat rheumatoid arthritis, leukaemia and multiple sclerosis.
When given the drug the men "went down like dominoes", vomiting, screaming in pain and breaking out in fevers, according to one witness who was given a placebo in the tests.
TeGenero said the drug, TGN1412 was a monoclonal antibody that stimulates more white blood cells known as T-lymphocytes to be produced.
While immunologists throughout the world knew drugs targeting molecules on T-cells must be developed with care, pre-clinical evidence suggested TGN1412 was safe and it was seen as an important development in this area, the company said.