Dow Chemical Co.'s plan to buy Union Carbide Corp. could prompt an overhaul of the Dow Jones industrial average, the most widely known benchmark for the U.S. stock market.

Union Carbide, one of 30 stocks in the average, today agreed to be purchased by Dow Chemical for $11.6 billion in stock and debt. Dow Chemical isn't in the average.

Dow Jones & Co., which owns the 103-year-old average, could simply add Dow Chemical to the benchmark. The financial publisher also could choose to add a big computer-related company such as Intel Corp. or Microsoft Corp. But Dow Jones plans no announcement any time soon.

"Any time there is an external event like this, it's an occasion to review the whole roster," said John Prestbo, markets editor of the Wall Street Journal. Prestbo oversees the average for Dow Jones, which publishes the Journal.

Such a review is already underway, he said, because of changes taking place at two of the Dow 30: Hewlett-Packard Co. is spinning off its unit that specializes in medical equipment and electronic measurement, and Exxon Corp. is acquiring Mobil Corp.

There is a precedent for keeping the merged entity in the average when a company that's not in the Dow industrials buys a company that is, Prestbo said. In 1985, Philip Morris Cos., which wasn't in the average, acquired General Foods, which was. Dow Jones added Philip Morris to the industrial average.

For the companies involved, inclusion in the Dow 30 is a matter of prestige. It's a sign that the company is among the most important in its industry. "We don't know if we'll be part of the Dow Jones average, but we'd be pleased if we were," William S. Stavropoulos, Dow Chemical's president and chief executive, said during a conference call with reporters.