After taking part in a clinical trial of a new drug six men are now seriously ill in a north London hospital in the UK.
Health officials say the volunteers became ill after taking a drug being developed to treat chronic inflammatory conditions and leukaemia.
Relatives are apparently at the bedside of the patients who apparently suffered multiple organ failure. Some lives are said to be in danger.
The trial was immediately suspended as the patients were admitted to Northwick Park Hospital after reacting badly to the drug.
According to the Medicines and Healthcare products Regulatory Agency (MHRA), a medicines watchdog, eight men took part in the trial, two were given a placebo and the other six who took the drug became ill.
U.S. drug research company Parexel International set up the trial for German pharmaceutical company TeGenero.
Professor Herman Scholtz, head of Parexel International Clinical Pharmacology, has said they are assuming that the adverse affects were a drug reaction which occurs extremely rarely and is both unfortunate and unusual.
According to the drug company those who became ill were taking part in the first human tests of an experimental drug called TGN 1412 which is being developed by TeGenero to treat diseases such as cancer and rheumatoid arthritis.
Parexel says all regulatory, clinical and medical guidelines had been followed during the trial.
The MHRA watchdog says it has suspended the trial and notified other European regulatory bodies about the incident in order that no further patients are harmed.
An exhaustive investigation to determine the cause and ensure all appropriate actions are taken has been promised by Kent Woods, MHRA's chief executive officer.
The unnamed patients were moved to a critical care unit at the Northwick Park Hospital in north London on Monday from a research centre on the same site.