STOCK PRICES UP IN MIDDAY TRADING

Stocks rose today, led by computer-related shares, as Yahoo!Inc., the leading Web-search service, became the second Internetcompany chosen to join the Standard & Poor's 500 Index.

At noon, the Dow Jones industrial average was up 75.31 points to10,953.12. The Nasdaq Composite Index, half computer-related shares,rose 29.17 to 3,365.33. The S&P 500 Index climbed 8.23 to 1,397.11.Three stocks fell for every two that advanced on the New York StockExchange.

Yahoo! surged 161/8 to 2287/8. The company will replace LaidlawInc. in the S&P 500 after the market closes Dec. 7, becoming thesecond Internet company after America Online Inc. to join the index.

New additions to the S&P 500 typically rise because mutual fundsthat try to mimic the performance of the index must buy the shares.Money managers often are judged by comparing their performance withthat of the S&P 500. About $750 billion is invested in funds thattry to track the performance of the S&P 500.

Laidlaw fell 5/8 to 51/2, and was the biggest loser in the S&P500.

Henry Blodget, an Internet analyst at Merrill Lynch & Co.,recommended Yahoo, America Online, Doubleclick Inc. and Amazon.comInc.

America Online gained 21/8 to 75, Doublclick gained 9 3/16 to1691/4, and Amazon.com rose 2 9/16 to 875/8.

Ardent Software rose 4 13/16 to 31 1/16 after the company agreedto be acquired by Informix Corp., the No. 3 database software maker.Ardent holders will receive $38.50 a share, or $880 million, inInformix stock. That's a 47 percent premium to Ardent's closingprice Tuesday. Informix fell 3/8 to 105/8.

General Electric gained 31/2 to 1331/2 after USA Today reportedGE's NBC Television Network and Microsoft Corp. are considering aninitial public offering of their joint MSNBC cable television andInternet news partnership. MSNBC competes with Bloomberg Television.Microsoft rose 1 5/64 to 921/8.

Atlantic Richfield fell 43/4 to 91 3/16 and BP Amoco's Americandepositary receipts fell 5/16 to 605/8. U.S. Federal TradeCommission staff lawyers recommended challenging the BP Amoco-Arcocombination, setting the stage for a court fight in the next twomonths.

"By virtue of the vast consolidation in the group and judging bycomments in (a New York Times article), this could be a wrench inthe works," said Bryan Piskorowski, a market analyst at PrudentialSecurities Inc.