Data compiled by Lombard Street Research shows that the M3 ''broad money" aggregates fell by almost $50bn (£26.8bn) in July, the biggest one-month fall since modern records began in 1959.
"Monthly data for July show that the broad money growth has almost collapsed," said Gabriel Stein, the group's leading monetary economist.
On a three-month basis, the M3 growth rate has fallen from almost 19pc earlier this year to just 2.1pc (annualised) for the period from May to July. This is below the rate of inflation, implying a shrinkage in real terms.
The growth in bank loans has turned negative to a halt since March.
"It's obviously worrying. People either can't borrow, or don't want to borrow even if they can," said Mr Stein.
Monetarists say it is the sharpness of the drop that is most disturbing, rather than the absolute level. Moves of this speed are extremely rare....
Monetarists insist that shifts in M3 are a lead indicator of asset prices moves, typically six months or so ahead. If so, the latest collapse points to a grim autumn for Wall Street and for the American property market.
As a rule of thumb, the data gives a one-year advance signal on economic growth, and a two-year signal on future inflation.
"There are always short-term blips but over the long run M3 has repeatedly shown itself good leading indicator," said Mr Stein...
M3 surged after the onset of the credit crunch, but this was chiefly a distortion caused by the near total paralysis in parts of the American commercial paper market. Borrowers were forced to take out bank loans instead. The commercial paper market has yet to recover