Labelled as "the mould which makes penicillin", the green substance was produced by Fleming in his laboratory after he discovered penicillin in 1928.
The Ayrshire-born bacteriologist went on to the win the 1945 Nobel Prize in medicine for the discovery which ultimately revolutionised medicine.
The samples were sold at Bonhams in London for £24,375.
It was while studying influenza that Sir Alexander famously noticed mould had developed accidentally on a set of culture dishes being used to grow the staphylococci germ.
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The mould had created a bacteria-free circle around itself. Sir Alexander experimented further and named the active substance penicillin.
However, it was two other scientists - Australian Howard Florey and Ernst Chain, a refugee from Nazi Germany - who developed penicillin further so that it could be produced as a drug.-read more