That's a breaking of the ranks since Burger King and other chains earlier said they wouldn't respond to McDonald's strategy.
Two TV spots from Ammirati Puris Lintas, New York, broke last week promoting the 99-cent Whopper with "no strings attached," a jab at the McDonald's caveat requiring customers to purchase fries and a drink to get the 55-cent sandwich.
A Burger King spokeswoman said the four-week price promotion was on the marketing calendar before McDonald's announced its plans.
The BK ads broke several days before McDonald's shifted its ad focus from Egg McMuffins to 55-cent Big Macs.
BK also is running national ads supporting $2.49 Whopper Jr. combo meals.
"Campaign 55" so far appears to be raising traffic but not sales for McDonald's, said analyst Damon Brundage, NatWest Securities Corp.
"I've noticed our volumes are down" since "Campaign 55" started, said a Burger King franchisee who operates 30 stores in the mid-Atlantic region. Multiunit franchisees in Florida and Wisconsin said their sales have not been hit by McDonald's.
The burger chains enter their next phase of competition in late May, with competing summer movie tie-ins. McDonald's will support Walt Disney Co.'s