An old mould linked to British scientist Sir Alexander Fleming's research which led to the discovery of penicillin is set to fetch thousands of pounds at auction next month.
The specimen, which is mounted on paper signed by Sir Alexander, was part of ground-breaking work which revolutionised medicine and went on to save millions of lives worldwide.
It is being sold at auction with two letters: one from the Scots-born, Nobel Prize-winning scientist and the other from Elizabeth Montgomery, believed to be his housekeeper.
In 1955, the pair wrote to a Mr and Mrs Bax, who lived opposite Sir Alexander, to thank them for their help when they saw burglars trying to break into his home.
Ms Montgomery, who lived next door to Sir Alexander, included the specimen, Penicillium Notatum, with her letter of thanks as a "souvenir of the Fleming family".
Her letter ended: "As though you didn't know - but just in case - this said affair is a blob of the original mould of penicillin, not to be confused with Gorgonzola cheese!!!"
Sir Alexander's letter, hand-written by him in blue ink on pale blue paper, is dated March 8, 1955 - just three days before he died at his London home following a heart attack.
The son of Mr and Mrs Bax, who declined to be identified, said: "I remember that my father often told the story of how he and my mother helped the Flemings and how excited they were to receive the thank you letters and the specimen, linked to such a significant medical discovery.
"The penicillin was often shown to family and friends and my parents had been so surprised when they found out that they lived opposite such an eminent scientist.
"Perhaps it had some influence on us children as my sister and I both ended up with careers in medical science, who knows."
The specimen and the two letters are estimated to sell for up to £6,000 at Catherine Southon Auctioneers at Farleigh Court Golf Club, Surrey, on July 8.