ECONOMY T-bill yields fell. Yields for 52-week bills dropped to 7.25 percent at the Treasury's monthly auction, the lowest since 7.21 percent Jan. 11. The average investment rate was 7.78 percent. Assets of the nation's 524 money market mutual funds rose by $1.72 billion to $408.76 billion in the week ended Wednesday, the Investment Company Institute reported. The Lipper Analytical Services index of the 30 largest growth mutual funds was 433.86, off 1.76 percent.

COMPANIES Shulton Inc. of Clifton, N.J., plans to slash 680 plant jobs in the wake of parent company American Cyanamid's decision to sell off Shulton's consumer products lines.

ACQUISITIONS American General, a Houston-based financial services concern that has tried unsuccessfully to find a buyer for the whole company, said it may settle for selling only its large insurance unit. Interpublic Group of Companies Inc., the giant advertising holding company, said it is offering to buy the 64.6 percent interest it does not own in the British-based ad company, The Lowe Group PLC. The deal would be worth about $165 million in cash and stock. Martin Marietta sold 150 acres of land for hotel and retail development in Orlando, Fla., to three groups of investors for $42.5 million. Pathe Communications made its final $50 million down payment for MGM/UA Communications in a move that may cut into the widespread industry skepticism that it can complete the $1.3 billion deal. The down payment means that Pathe, controlled by Italian financier Giancarlo Parretti, has made $353 million in cash payments for the studio since announcing the deal last March. Philips NV, the Dutch electronics giant, said it will sell its 15 percent stake in a telephone equipment company set up jointly with American Telephone & Telegraph. AT&T will buy the stake, boosting its share in the joint venture from 59 percent to 74 percent.

INTERNATIONAL Czechoslovakia became the 152nd country to join the IMF, agreeing to contribute a quota of about $826 million into the fund, which now stands at about $127 billion. Air France said it will intensify a cost-cutting program in response to an estimated loss of $32 million in the first half of 1990 and worries about fuel price hikes in the second half. NEC Corp. of Japan revived plans to set up joint ventures in China to produce integrated circuits and digital telephone switching systems. The deal had been initiated in 1988 but was suspended after the Chinese government's crackdown on demonstrators in June 1989. Japan surpassed Britain last year to become the world's fourth-largest beer brewer, said Kirin Brewery, Japan's biggest brewery. The largest producer by far was the United States with 6.07 billion gallons, up 0.6 percent from 1988, Kirin said. West Germany was second followed by China, Japan and Britain.

REGULATION United received clearance from a federal appeals court to revamp its schedule at Chicago's O'Hare airport after Oct. 1. The change, which was challenged by American Airlines, had been approved by the FAA as a means of smoothing out evening arrivals and departures and easing congestion. United Bank of Philadelphia received charter approval from the Pennsylvania Department of Banking, helping to pave the way for a planned winter opening of the black-controlled commercial bank.

CONTRACTS Advanced Technology of Reston has been awarded a five-year, $10.9 million contract with the Naval Surface Warfare Center to provide engineering and technical support for the Navy's Combat Systems Laboratory and Aegis facilities.

EARNINGS National Semiconductor posted a $165.5 million first-quarter loss, largely because of a $143.6 million restructuring charge.

PEOPLE Jean-Pierre Cloiseau was appointed senior vice president and chief financial officer of Lafarge, the Reston-based construction materials manufacturer. Cloiseau succeeds Francois Jaclot, who returned to Paris to work for Lafarge Coppee.