Brazil, Latin America's biggest debtor, reached agreement with a commercial bank committee yesterday on a $31 billion refinancing package that significantly lowers interest rates it must pay.

The agreement follows Brazil's announcement Friday of a far-reaching program designed to reduce inflation, which in recent months reached an annual rate of 400 percent. It also comes amid growing demands by Latin American governments for interest-rate concessions to enable them to meet payments on the region's $370 billion debt.

The agreement refinances $6 billion in debt that matured in 1985 and $9.5 billion that matures in 1986 and provides about $15.5 billion of trade and other financing. The bankers agreed to lower the interest rate on the $15.5 billion of maturing debt by nearly one percentage point, which will save Brazil about $150 million a year, officials said.

Brazil, which had pressed its creditors to lower their interest-rate spreads, or markups, during the month-long negotiations, was given the more favorable financing terms by the 14-bank advisory committee as a reward for managing its finances better, the committee said.

The agreement with Brazil was seen by bankers yesterday as an endorsement of the way the creditors are handling the debt problem.

It also serves as an example for other financially strapped countries at a time of growing tension between lenders and debtor nations demanding lower interest rates, particularly for countries hurt by plunging oil prices, bankers said.

The agreement announced yesterday reflects "significant progress by Brazil over the past few years on its external financing accounts," said William R. Rhodes of Citibank, the committee chairman.

Brazil had trade surpluses of $13.1 billion in 1984 and $12.45 billion in 1985, according to the bank committee. Brazil also has raised $9 billion in its foreign cash reserves since the end of 1983, when the account was in the red.

The proposal will go to the 700 international creditor banks involved with Brazil's debt for their approval. An agreement is expected to be signed by June, Rhodes said.

Brazil owes commercial banks about $66 billion of its $102 billion in total foreign debt.

Last week, officials of major Latin American debtors meeting in Uruguay said that the key area in which many hard-pressed countries need prompt relief is in the interest rates and in the "spreads" -- markups that banks add to their cost of funds when making loans.

Banking sources said that the interest-rate spread was a key topic of discussion during the month-long negotiations on Brazil's new debt restructuring package.

Under the agreement announced yesterday, Brazil will pay 1 1/8 percentage points above the London Inter-Bank Offered Rate, a benchmark rate on international loans. It had been paying an average of two percentage points above the London rate.

On Friday, Brazilian President Jose Sarney announced broad-based economic changes, including an end to indexing of wages, a price freeze and a new monetary unit in an effort to slow its rampant inflation.

Western bankers have urged Brazil strongly to take tough measures to reduce its inflation rate. But banking sources also said that the timing of the debt rescheduling, the calls for lower interest rates by the Latin debtors and Brazil's new inflation-fighting plan were coincidental.

Bankers said that the Brazilian agreement has no immediate significance for more hard-pressed debtors such as Mexico because each case will be taken on an individual basis, depending on how much progress the country makes in combating internal economic difficulties and managing its debt.