By summer the U.S. Export-Import Bank may be active in virtually all of Eastern Europe, including the Soviet Union, John Macomber, the bank's chairman, said Thursday.

Mr. Macomber's comments suggested that the U.S. export financing agency will open for business soon in East Germany, Romania and Bulgaria, besides the Soviet Union.The bank recently announced that it will resume financing exports to Czechoslovakia and Poland. It had already been financing U.S. sales to Yugoslavia and Hungary.

Meanwhile, it was learned, an effort will be made in Congress to boost Ex- Im Bank's lending limits next fiscal year to $750 million, 50 percent more than the Bush administration has proposed.

And Rep. Walter E. Fauntroy, D-Del., the chairman of the House Banking subcommittee that oversees Ex-Im Bank, said he will try to force the administration to implement a program of Ex-Im Bank interest rate subsidies on commercial bank export loans.

The subsidy program, known as interest equalization, was authorized by Congress last year to help boost Ex-Im Bank's export financing capability.

Every dollar of Ex-Im Bank subsidy would support $10 of U.S. exports, it is estimated.

But Richard Darman, director of the Office of Management and Budget, has refused, for budgetary and perhaps philosophical reasons, to let the program take effect.

Mr. Fauntroy said his new bill would require the administration to implement the subsidy program.

Both Mr. Macomber's and Mr. Fauntroy's comments came at a one-day conference here sponsored by Ex-Im Bank.

The push for a $750 million Ex-Im Bank direct lending program in fiscal 1991 has the support of the chairman of the House Banking Committee, Rep. Henry Gonzalez, D-Texas, congressional sources said.

The proposed funding increase partly reflects new export opportunities opening in Eastern Europe. Last year, Congress provided $612 million for Ex-Im Bank.

The proposed increase, however, is certain to be resisted by the administration. Last week, Mr. Darman reportedly rejected a new Ex-Im Bank plea for more than $500 million in direct lending authority next fiscal year.

Mr. Macomber, however, held out the prospect of substantially more export financing for Mexico this year, under a new scheme called bundling.

Bundling is the combining of a number of relatively small export credits, guaranteed by Ex-Im Bank, into a large overall credit line, which is then offered to capital markets. By year's end, Mr. Macomber said, at least $500 million in U.S. exports to Mexico may be financed in this way.