Scotland was used as a testing ground for weapons containing bubonic plague, according to secret defence papers which have been made public.
The tests were carried out by Porton Down scientists
A fishing boat crew from Fleetwood in England was accidentally exposed during testing and then covertly monitored.
The exposure happened during MoD tests of biological weapons in 1952.
Pontoons containing live monkeys and guinea pigs were moored off the coast of Lewis and clouds of bubonic plague were exploded above them.
But a trawler unwittingly steamed into the danger zone and Ministry of Defence ships shadowed the craft for weeks to monitor any emergency calls about the health of the crew.
All documents about the affair, apart from a sanitised Admiralty report, were ordered to be destroyed and the incident remained secret until now.
The tests were part of a biological weapons research programme based at Porton Down in Wiltshire conducted between the second world war and the mid-1950s.
However, islanders said the tests were an open secret locally at the time.
Local councillor Angus Nicholson said: "The animals were put on a pontoon about a mile from the mother ship and then weapons were exploded around these animals to see how easy it would be for them to catch some very noxious diseases."
Doctor William Balmer, of University College London, a specialist in biological weapons, said the bubonic germ was exploded from a boom attached to the pontoon.
He added: "It was drifted onto the monkeys and guinea pigs and from then onwards it was the job of the scientists to see what happened."
Former fisherman, Derek Bellerby, recalled seeing the trawler.
"We were homeward bound and we saw this ship signalling and trying to contact the skipper to no avail," he said.
"The ship came up and reported that we were in a restricted zone."
Angus Brendan MacNeil, the MP for the Western Isles, said the testing exercise was "staggeringly incompetent".