The Justice Department has started a preliminary investigation into whether I.B.M. has abused its monopoly position in the market for mainframe computers, which remain vital to many of the world’s largest businesses. This month, antitrust regulators at the Justice Department began seeking information about I.B.M.’s business practices from companies that compete with I.B.M. in the market for large computer hardware and software, people who had been contacted in the inquiry said.

The requests for information followed a complaint filed by the Computer and Communications Industry Association, a trade group with a history of involvement in antitrust disputes. The organization, which is backed by I.B.M. competitors like Microsoft and Oracle, contends that I.B.M. stymied competition in the mainframe market and blocked efforts by competitors and potential partners to license I.B.M.’s software.

The complaint follows similar legal action taken by T3 Technologies against I.B.M.

T3, a small company that resold mainframelike computers, filed an antitrust complaint against I.B.M. in January in Europe. T3 also filed a civil suit against I.B.M. in the United States. Last week a federal district judge in New York dismissed that case. T3 said it planned to appeal.

Steven Friedman, the president of T3, said he had received a formal request for information from the Justice Department about I.B.M.’s actions in the mainframe market. “They asked for a very broad set of documents and information,” he said.