Boston Properties, the American owner of office buildings founded by Mortimer Zuckerman, agreed to sell 280 ParkAve. in Midtown for about $1.2 billion to take advantage of rising property prices.

Boston Properties isn’t identifying the buyer of the 1.18 million-square-foot complex, which comprises a 30-story building and a 43-story tower, Michael Walsh, senior vice president of finance, said in an interview yesterday. The transaction probably will close by June.

The Boston-based real estate investment trust last month said it was in talks to sell two Manhattan properties and would use the proceeds for acquisitions, cutting debt, buying back shares, or paying a special dividend. It hasn’t yet found a buyer for the other property, the 37-story 5 Times Square.

“These assets are all of the highest quality and one for which there is a worldwide demand,” Mr. Zuckerman, 68, said on a conference call yesterday with investors and analysts.

Istithmar PJSC, a state-owned investment company based in Dubai, said last week it wouldn’t purchase 5 Times Square because of tax concerns. While Mr. Walsh wouldn’t confirm Istithmar was interested, he said a deal fell through because of problems with the independence of the proposed buyer’s auditor. The auditor, Ernst & Young International, occupies the entire building.

Boston Properties sold 280 Park Ave. to take advantage of rising prices for office buildings in Midtown, which has the highest rents in America. The Park Avenue complex fetched $1,018 a square foot, the highest price for which Boston Properties has sold a site, Mr. Walsh said.

Boston Properties will continue to manage the site for the buyer, the chief financial officer, Douglas Linde, said on the conference call.

Net proceeds from the sale will be $850 million, Mr. Walsh said. The transaction will reduce funds from operations, a measure of cash flow used by real estate investment trusts, by 15 cents a share on an annual basis, he said. That impact isn’t included in the company’s 2006 funds from operations forecast of $4.15 to $4.27 a share.

The company last night reported that first-quarter net income climbed to $67.7 million, or 59 cents a share, from $61.2 million, or 55 cents, a year earlier. Funds from operations were unchanged at $1.03 a share.

Boston Properties shares fell 46 cents to $87.31 as of 12:07 p.m. in New York Stock Exchange composite trading. They’ve gained 37% over the past year, compared with the 20% increase in the Bloomberg Real Estate Investment Trust Index.