THE EXPERTS say that the prime is not the most significant of the interest rates. It is set by banks as a matter of policy, the explanation goes, and is far less sensitive to the shifting financial currents than the rates set by auction in the marketplace. Perhaps so; but whatever the prime rate may lack in sensitivity it more than makes up in visibility. Yesterday most of the big banks raised their prime rates-- the basic lending rates to the biggest and best customers--from 101/2 percent a year to 11 percent. That's a significant change, and it's in the wrong direction.

It's a response to the slow but steady rise in other rates--the rates at which banks borrow--over the past three months. When the banks dropped the prime to 101/2 percent last winter, it was still more than two percentage points above the banks' key borrowing rates. But by last week their margin had been squeezed to less than one percentage point and an increase in the prime was inevitable.

As for the timing of the increase, it is obvious that the bankers were waiting for Congress to leave town and disperse itself across the landscape. Congressional reaction, however outraged, always has a muted and diffused quality when it comes floating back from beaches, mountains and the capitals of Western Europe. The bankers were wise to wait. Congress has been working on much legislation that concerns the banking industry. Only a few days ago President Reagan signed into law the repeal of withholding of income taxes on interest and dividends. The crucial IMF legislation is still very far from completion, and much of Congress thinks of it as a bankers' bill. That's an error, but the bankers have enough of a stake in it to have wanted to avoid offending congressmen gratuitously while it was moving slowly and with very narrow margins through the House last week.

By raising the prime, the banks were also offering a judgment that the recent rise in interest rates was not going to reverse itself very soon. On hearing that opinion, the stock market fell down the stairs again. It is better for the economy, and the people whose livelihoods depend on it, when the see-saw moves the other way--stocks up, rates down.

If Congress, on its return in September, wanted to do something about the rates, where might it look? Legislated ceilings on rates don't work. Interest subsidies are much too expensive. But there is one thing that comes to mind. People here in Washington have begun to say that the discussion of the deficit has become repetitive and a bore. How true. But however boring it may be, and unappealing to people of lively intellect, that large and growing deficit continues to be important. Shrinking it is essential to any real remedy of the things going wrong in the world of money.