The Supreme Court Monday refused to consider a significant antitrust case involving Northwest Airlines and a low-cost competitor that could have set some rules for competition between large airlines and smaller, no-frills rivals.

The case, Northwest Airlines Corp. v. Spirit Airlines Inc., stems from a six-year-old Spirit Airlines suit in which Spirit charged that Northwest engaged in predatory pricing to force Spirit to drop two flights from Detroit, one to Boston, the other to Philadelphia. The case raises key antitrust issues, such as the type of evidence needed to prove that a company engaged in below-cost pricing to drive a competitor out of business.