A handful of veterans, involved in secret military research in Wiltshire in the 50s, have been invited back to see around the laboratories.
An inquest ruled Ronald Maddison was unlawfully killed
Many claimed they were fooled into taking part in the trials at Porton Down which later damaged their health.
Former airman Douglas Shave believes the nerve gas tests he underwent in 1950 left him with severe eczema.
He was exposed to the toxin, sarin, but experts at Porton have told him his skin complaints are unrelated.
A mix-up over his notes led him to believe he had been inside a sarin chamber for more than an hour.
"I do believe that if I hadn't have gone there, I don't think I'd have had the problems I've had over the years," he said.
During his return visit to the laboratories, the director of Porton Down, Rick Hall, apologised to Mr Shave, admitting that there had been a mistake.
"The original person who looked at the records read it as one hour and forty-six seconds, when in fact it's one minute and forty six seconds," he said.
In November 2004, an inquest into Ronald Maddison, who died 51 years ago during secret nerve gas tests, ruled that he was unlawfully killed.