Half a dozen lawmakers, including New York Sens. Hillary Rodham Clinton and Charles E. Schumer, have asked for an investigation of cheese trading at the Chicago Mercantile Exchange, which affects milk prices nationwide.

Noting that concerns have been raised in meetings with farm groups, the senators asked the Government Accountability Office to consider whether the CME’s market fairly represents supply and demand for milk or is simply a leverage for big companies to influence prices farmers are paid for milk.

“There has been concern in the past that this ‘thin’ market for cheese is susceptible to price manipulation because the price of essentially all cheese is linked to less than 1 percent of the cheese that is actually delivered in the cash market,” the senators wrote to the GAO, the investigative arm of Congress.

Others signing the letter were Sens. Arlen Specter, R-Pa., James Jeffords, I-Vt., and Herb Kohl, D-Wis.