Riggs Bank took the unusual step yesterday of cutting its prime lending rate without waiting for the lead of the big New York banks that usually set trends in interest rates.

No major banks followed Riggs in cutting the prime from 9 percent to 8.75 percent. Economists and executives of other financial institutions said Riggs' move might be premature, but was not a publicity stunt.

In the last two months the prime, the rate charged to banks' biggest and most creditworthy customers, has gone up three times and down twice during a period of unusual volatility in financial markets.

"Riggs' decision may be based more on expectations, while the rest of the banks are waiting for the market to settle down a little bit," said Edward Boss, economist for Continental Illinois National Bank in Chicago. Based on the rates banks are paying for money, lending rates had been expected to come down toward the end of the year, he said.

"By no sense is it out of line," said F. Ward McCarthy, chief financial economist for Merrill Lynch & Co. Inc. in New York. "I don't think there will be anybody following them immediately, but I think in three or four weeks you might see some others."

Riggs' cross-town rival, American Security Bank, said it was not cutting its prime rate immediately, nor was its parent, Maryland National Corp. of Baltimore. "Both American Security and Maryland National usually move the prime based on several trending factors," said spokesman Roger Conner. "One local bank moving the prime down would not be a trending factor."

The prime rate traditionally follows a nationwide pattern set by big New York banks that specialize in business loans. But even the trend setters sometimes announce changes that are not followed. Last month Chemical Bank and Marine Midland both boosted their prime from 9.25 percent to 9.75 percent, only to pull back a few days later when other banks didn't follow.

Some smaller banks occasionally seek the limelight by jumping the gun on prime rate changes, but Riggs has never before played that role.

Riggs Senior Vice President David Bunting said the bank decided to reduce its prime because it had experienced an influx of funds since investors began pulling out of the stock market and because the interest rates paid depositors were falling.

The average rate paid by banks on certificates of deposit has come down a full percentage point, he noted, and there has been a similar decline in the federal funds rate charged on overnight loans to banks by the Federal Reserve Board.

"We felt now was the time, based on our cost of funds," said Bunting. In terms of the rates charged on loans, he added, Riggs executives decided, "If we were a little premature, we might stimulate some business."

Riggs' cut in the prime "may prove to be a smart business decision," said William Sullivan, a Dean Witter Reynolds Inc. money market economist.

Riggs' cut will mostly benefit the real estate developers who are the banks' biggest customers, Bunting said. It will not affect rates charged on Riggs' home equity and executive credit lines, even though they are linked to the prime.

He explained that the prime rate on which those loans are based is the one published in The Wall Street Journal, an average of big city banks.

He said Riggs doesn't use its own prime rate for determining rates on those loans because of federal truth-in-lending laws, which specify that variable rate loans be linked to factors easily measured by consumers. Since Riggs isn't traditionally a prime-setter, its prime rate doesn't meet that criterion.