The Federal Trade Commission yesterday moved to block Dun & Bradstreet's proposed $446 million purchase of Information Resources Inc., saying it would "substantially reduce" competition in the market for syndicated tracking services.

Syndicated tracking is a relatively new and highly specialized form of market research that uses such methods as computer scanning at checkout counters to track the products consumers are buying. Only three companies are currently in the market, with Dun & Bradstreet's A.C. Nielsen subsidiary accounting for about 38 percent of the $200 million a year business.

The proposed merger with Information Resources would give Dun & Bradstreet about 51 percent of the market, a commission official said. The third company in the business is SAMI-Burke, a subsidiary of Time Inc.

The FTC, under its current chairman, Daniel Oliver, has been criticized for failing to use the antitrust laws to block mergers potentially damaging to consumers. But commission staffers noted that yesterday's action was the sixth time this year the agency has moved to stop an acquisition. The five commissioners voted unanimously to block the IRI acquisition, they said.

Dun & Bradstreet announced its plans for the acquisition of the Chicago-based Information Resources on Aug. 27, saying it would swap about 8.54 million shares of its common stock for all outstanding shares of IRI. At the time, the deal was estimated to be worth about $572 million.

Since the Oct. 19 stock market crash, however, the value of Dun & Bradstreet's stock has dropped, reducing the value of the acquisition about 20 percent. D&B stock closed yesterday at $53.25, up $1.50, on volume of 243,500 shares.