Defence scientists secretly tested E.coli bacteria as a possible biological weapon near two UK towns.
The report concerns Porton Down's MoD labs
Trials were carried out near Swindon and Southampton between 1965 and 1967 involving the release of "microthreads" covered in the bacteria.
The tests are detailed in the 1966 MoD report on the laboratory at Porton Down, Wiltshire, just released.
They were designed to see how well the bacteria would survive in different climatic conditions.
The documents are available for viewing at the National Archives in Kew and discuss the "production of micro-organisms for weapons systems".
There appears to be no mention of whether the bacteria infected anybody during the trials.
The documents say that the "excellent quality and reproducibility" of E.coli indicated "highly satisfactory results" could be achieved.
In a statement the MoD said: "This was not a test of weapons or a harmful substance. It was to help the UK to assess the risk of an attack using biological agents by providing important information about their likely dispersion in the open air during the Cold War.
"Bacillus globigii var. subtillus (BG) is common a natural substance in the environment. None of the senior Medical Microbiologists that Professor Spratt consulted during an independent review believed that the inhalation of even a million BG spores constituted any significant threat to health."
Preliminary experiments were conducted in 1965 in and around Southampton, with follow-up tests the next summer.
Similar experiments were performed in Swindon, which was selected "because it is a reasonably large inland industrial city in the midst of a large rural area".
Another 12 trials were conducted in November and December in Southampton to see if there was any relationship between pollution and the survival of E.coli.
Five stations were deployed in each trial: one upwind of the city, one in the centre, and three at different distances downwind, the report said.
The report says that no cases of laboratory infections had been detected at the Porton Down labs in 1966, while other safety activities were "too trivial to deserve special mention".