Lot 92 at Bonhams auction house isn’t much to look at—just a blob of mold encased in glass and plastic. But this mold is famous. It’s part of the original culture of Penicillium chrysogenum that led to bacteriologist Alexander Fleming’s 1928 discovery of the antibiotic penicillin.

Fleming preserved the specimen himself in his lab. It’s expected to sell at a March 1 auction for at least £4,000 (about $5,000), a price that reflects a surprising abundance of historic mold on the market.

When Sotheby’s offered a similar mold medallion in 1996, it was believed to be one of only a very few surviving specimens. The drug manufacturer Pfizer bought it for £23,000 (about £41,000 today, or $51,000).

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