If you've a fiver in your wallet - double check it now - as it could be worth a huge fortune.
Collectors are paying hundreds of pounds for the very first batch of banknotes that went into circulation last September - with rare editions fetching anything from double the value to £80,000 on auction.
The most highly prized are those with the serial code beginning in 'AA01'.
If you've a good eye - you'll be able to spot the special ones when they land in your wallet.
One lucky eBay seller - who happened to find two - proudly sold a freshly minted polymer £5 note as "Brand New. Uncirculated. AA01 Serial Number" for £227 on the auction website.
The same seller traded away a similar note for £215 just days later.
Valuable £5 notes - the signs to watch for
The closer the banknote is to the very first batch produced, the more valuable it could be.
These are the notes that start with the 'AA01' prefix. If your note is then followed by a serial code in the low digits, it's likely to be even more attractive.
Unfortunately, once the banknote serial number leaves the vaunted 'AA01' domain its value among collectors slumps dramatically, although there are examples of banknotes with the 'AA' prefix being sold for around £20 online.
A handful of limited edition £5 notes stamped with a Jane Austin engraving are also in circulation - and these are said to be worth as much as £20,000 - with bidders willing to pay as much as a house despot for the prized posession.
Don't forget, the new tenner is also on its way. Here's what you need to know about the new plastic £10 note - including the valuable signs.
I've not got a AA01 note - but is my fiver still valuable?
Even if your freshly minted £5 notes don't come close to the prized 'AA01' territory, it is worth nothing that some collectors also prize sets of banknotes with sequential serial numbers.
The Bank of England will be auctioning off a complete set of 60 £5 banknotes, with serial code prefixes ranging all the way from 'AM01' – 'AM60'; where they are expected to sold for between £3,000 - £4,000, more than ten times their nominal value.
One man, Alan Scrase, managed to flog three £5 notes for £460.
The 52-year-old sold the money on eBay.co.uk, and has revealed why he managed to see such a welcome return.
Alan went to a local bank branch just after a delivery on the polymer bank notes . He visited a day after the cash launched.
And Alan managed to get his hands on three of the hardy plastic notes with consecutive AA01 serial numbers.
It's "the luck of the draw," he said.
An avid collector, Alan had not managed to get any new fivers on launch date.
"I did not get them on 13 September - the first day - but it was just by sheer chance that I went to the bank the next day and they had just got them in," he said.
"You just go in your bank and ask for them. I got a few."
Having timed his bank request to perfection, Alan went on the internet auction site to sell up – and said that he's going to reinvest his earnings on his hobby.
He saw 31 bids on his £15 set. The top came in at £456.
"I am surprised how much they have gone for," he told BBC News .
"Any collector wants the first issue but they seem to have gone up in value very quickly."
The new fiver - all you need to know
This is the first time the Bank of England has issued plastic money – waterproof and far less likely to tear. A total of 440 million have been printed.
The very lowest serial number notes are given to the Queen.
For those of you who haven't been flogging your fresh fivers on eBay, there's a nice little experiment to try with a laser.
As scientist Steve Mould explains in the video above, if you shine a laser through the transparent Queen's head, with the cash held in front of a wall, a "triangular star field" appears behind.
Steve wonders in the clip whether the dot pattern is a deliberate security feature – as is the case with some bank notes, where the value of the money appears if you do the same trick.
He's not too sure. But either way, it's an interesting new finding, and is probably down to the way the Queen's face is printed on the polymer.
Steve says the star system comes due to a triangular grid of dots. When projected onto a wall, they look quite cool.
Not got a lucky fiver in your stash? Check our guide on the most valuable coins here.
You can also read all the gossip on the new plastic £10 notes and the upcoming £20 polymer notes here.
