Martin & Stern, a Vienna-based computer company embroiled in a tangled takeover dispute, filed for federal bankruptcy court protection. Its parent company, Systems Exploration Inc., was to be acquired by BTG, another Vienna-based computer systems company. But two lawsuits filed against MSI by Olin Corp. of Stamford, Conn., have stalled the deal.

Ernst & Whinney's New York offices have been barred for 45 days from taking on new clients who would have dealings with the SEC. An administrative law judge upheld a 1985 SEC finding that the accounting firm engaged in "improper professional conduct" in connection with audits of U.S. Surgical Corp. in the early 1980s.

Shearson Lehman Hutton said it was fined $500,000 by the New York Stock Exchange in connection with transactions in 1986 relating to the company's proprietary account.

Standard & Poor's is considering lowering credit ratings for Signet and Crestar out of concerns about the banks' exposure to a deteriorating real estate market. S&P said it is unlikely that any downgrade would drop below investment grade rating.

UAL, parent of United Airlines, halted talks for a $15 billion aircraft purchase at the request of the employee group trying to buy the company.