Fifty restaurants in Boston threatened to stop accepting the American Express card if the company refused to reduce the commission it charged on each purchase. Visa International, one of American Express’s rivals, offered to pay the group’s legal expenses. American Express then lowered its commission. For restaurants that processed more than $10 million in annual volume, American Express reduced charges from 3.25 percent to 2.75 percent. Rates for restaurants processing less than $10 million but more than $1 million a year fell to 3 percent.

The company did not lower rates for restaurants with volume lower than $1 million a year. Have either the restaurants, Visa, or American Express potentially violated the antitrust laws?