Tax officials are putting the frighteners on 50,000 plumbers, gas fitters and heating engineers.
They're being sent letters encouraging them to take advantage of an amnesty to pay tax on any income they may have neglected to mention.
Last year, the officials managed to prod 2,000 doctors and dentists into coughing up millions of pounds in unpaid tax.
One doctor admitted to owing £1m.
How could he owe so much? It was from private healthcare work, I understand, and the sum accumulated over a number of years.
And there would have been a penalty to include in the total of around 20% of the amount owed, under the amnesty.
It's the penalty which is the real clincher. After a full investigation Her Majesty's Revenue & Customs could have resorted to a 100% penalty and prosecution.
I am told that 70% would have been more likely in the doctor's case, so owning up would have saved him hundreds of thousands.
It seems unlikely that HMRC will extract so much from plumbers. In fact they have no idea how much they'll get.
The beauty of the amnesty tactic is that you just announce it, send out the letters and wait for the offenders to name themselves.
You only have to start working on investigations if you are unhappy with the results.
Who's next? I think solicitors and barristers should watch out.