American financial leaders reacted enthusiastically, at least publicly, to President Reagan's plan to cut personal and business taxes and reduce the growth in federal spending in an attempt to slow inflation, increase job-creating investments and lower unemployment.

The White House insists that its new economic program must satisfy the nation's financial markets if it is to be successful.

"I believe the president's program is an important first step on a long road to economic rehabilitation," said Willard Butcher, president of Chase Manhattan Bank, the country's third largest. "Perhaps more significantly, the president's speech signaled an important change of direction in national economic policy which I believe now demands the support of the country and the Congress. And we're with him."

"I'm heartened," aid Robert Baldwin, president of a big investment banking firm, Morgan Stanley & Co. Inc. "There's a lot of oxes being gored in this program. But you have to do it all at once."

Baldwin said he would have preferred it is Reagan had proposed reducing the maximum tax on unearned income (such as that derived from interest and dividends) to 50 percent from 70 percent, but that he recognized the president had to temporize on some proposals.

"I agree totally with the basic thrust of reducing spending and reducing regulatory costs," said Peter G. Peterson, chairman of the investment banking firm Lehman Brothers Kuhn Loeb Inc. and a former Secretary of Commerce in the Nixon administration.

However, he said, he thinks that the Reagan proposals neglected to deal with transfer programs such as government pensions and social security, which are adjusted each year to keep pace with inflation.

"From a dollars perspective as well as an equity and fairness perspective," he said, Congress and the President must address indexing of Social Security. He said recipients received a 14.2 percent tax-free increase last year although studies suggest that the cost of living for retirees rose about 8 percent or 9 percent. Those increases were financed by workers who recieved taxable wage increases of about 9 percent, which resulted in after-tax increases of 6 percent or 7 percent. As a result, social security recipients had a 6 percent real increase in benefits, while workers, who actually faced a 14 percent increase in their cost of living, had a real income decline.

"All in all, it's a good program. They're not emphasizing the fact that it's a long-range strategy, and they're underemphasizing the fact that the program provides the political base for the Federal Reserve to continue a policy of tight monetary restraint," said Steven Dobson, vice president and senior economist of Bank of America.

One of the general concerns in the business and financial community is whether the president will be able to shepherd his massive tax and budget program through a still-Democratic House of Representatives.

"I know the cynical side of Congress better than the next person when it comes to specific program," said John Perkins, president of Chicago's Contimental Illinois National Bank and a former chairman of the American Bankers Association. "But it has been a long time since I've seen the kind of consensus in the country.

"You talk to congressmen and you find they are all sensing the same thing: that the average middle-class person is a little desperate. Put it all together and, if ever there was a chance, this is it if he [Reagan] can capitalize on the consensus and keep leading properly."