NEW YORK, FEB. 17 -- Moody's Investors Service today downgraded the debt of several money-center banking companies that hold significant amounts of Third World loans.

The two companies most affected were Manufacturers Hanover Trust Co. and Bank of America, said Christopher Mahoney, associate director of Moody's.

Manufacturers' debt was lowered to Baa1 from A1, while its parent Manufacturers Hanover Corp. was cut to Baa3 from A3. Bank of America's rating was reduced to Baa2 from Baa1, while its parent, BankAmerica Corp., fell to Ba3 from Ba1.

In all, 11 companies and $82.6 billion of securities were affected by the move, although some ratings were confirmed as part of Moody's review of banks with large exposure to debt of less-developed countries, or LDCs. The review began Dec. 24.

Moody's action follows a similar move Feb. 1 by the other major credit rating agency, Standard & Poor's Corp.

"We took a new look at the overall LDC picture in the fall of 1987 based upon a sense that things were deteriorating, both in terms of the debt picture and the world economic outlook," Mahoney said.

Moody's decided "debtor discipline and creditor cohesion were deteriorating" and the "value of {Third World} loans was diminished and would continue to be diminished," Mahoney said.

He said recent moves by some debtors do not indicate the borrowing countries will be able to service their loans.

In the case of Brazil, which recently made partial interest payments after declaring a moratorium on long- and medium-term bank debt in February 1987, Mahoney said, "The money they are going to be {paying} is going to be money lent by the banks."

A plan by Mexico, which hopes to swap bonds for some of its debt, "is effectively forgiveness" because banks would have to trade a higher face value of loans than the bonds they would get in return, he said.

Mahoney said the 11 banks had Third World loan exposure spanning 80 percent to 180 percent of their equity. He would not specify the levels at the various banks, citing confidential information they provided to his company.

Other rating actions by Moody's: J.P. Morgan & Co. was lowered to Aa1 from Aaa; Morgan Guaranty Trust Co. of New York was confirmed at Aaa.

Bankers Trust New York Corp. was lowered to A1 from Aa3; Bankers Trust Co. was lowered to Aa2 from Aa1.

Citicorp was confirmed at A1; Citibank was lowered to Aa2 from Aa1.

Seafirst Corp. was confirmed at Ba1; Seattle First National Bank was confirmed at Baa1.

Irving Bank Corp. was lowered to Baa1 from A2; Irving Trust Co. was lowered to A2 from Aa3. The debt remains on review pending results of the hostile bid for the company by Bank of New York Corp.

First Chicago Corp. was confirmed at A2; First National Bank of Chicago was confirmed at A1.

Continental Illinois Corp. was confirmed at Ba3; Continental National Bank & Trust Co. was confirmed at Baa1.

Bank of Nova Scotia was confirmed at Aa2.

European American Bank senior debt was confirmed at A3 but remains under review.

The ratings specify the degree of risk investors assume by purchasing bonds and other debt securities issued by the banks. Generally, the higher a company's credit rating, the less it has to pay in interest charges when it borrows money in the bond market.