As the New York investment firm of Kohlberg Kravis Roberts & Co. completed its $25 billion acquisition of RJR Nabisco -- the biggest leveraged buyout in history-the takeover activities of KKR again came under scrutiny in Congress. KKR was criticized at a hearing yesterday of the House subcommittee on labor-management relations, which is looking into the impact of the LBO trend on pension funds and employment. Thomas R. Donahue, secretary-treasurer of the AFL-CIO, invoked the name of KKR frequently as he went through a list of companies where, he said, 90,000 union jobs had been lost "because of mergers, takeovers and leveraged buyouts." A spokesman for KKR said it doubted the accuracy of AFL-CIO figures and pointed to at least one factual error in the union's list. The union later acknowledged that KKR had not been involved in one of the deals cited. A day earlier, a former executive of RJR Nabisco told the House banking committee KKR and RJR Nabisco would have to cut employees and sell more pieces of the company than it had planned to pay the interest on the billions of dollars borrowed for the buyout. KKR said the claims were incorrect. Donahue, meanwhile, expressed concern about KKR's role in operating supermarkets. As a result of LBOs, he said, KKR "winds up as the single largest employer of workers in the retail food industry," supervising 165,000 to 175,000 of 600,000 union workers. Donahue said he worried about "the long-term effect" of people from the financial world running grocery stores. KKR's spokesman said he wondered about the source of the AFL-CIO figures. KKR representatives have been quietly making their case for the validity of LBOs to members of Congress and their staffs. KKR officials have declined to testify publicly. Henry Kravis, for instance, recently visited Sen. Lloyd Bentsen (D-Tex.), chairman of the Senate Finance Committee, and Rep. Dan Rostenkowski (D-Ill.), the chairman of the House Ways and Means Committee. Meanwhile, the subcommittee was told that a KKR study that was given to the panel's staff showed employment had increased at companies that were bought out by KKR. The KKR study was put into the hearing record. In New York, KKR attorney Richard I. Beattie said the investment firm was being represented in its visits to the Hill by members of Wunder and Diefenderfer, a Washington law firm. Beattie said KKR had been asked by several Hill staffs to talk to them about the leveraged buyout phenomenon and how it works. Beattie said KKR was concerned about the confusion on the Hill between mergers, acquisitions and leveraged buyouts. The issue that most concerns KKR, he said, was the tax deductibility of interest on corporate debts that accompany leveraged buyouts. The tax break available to corporations has been cited as a main cause of LBOs.