The Peabody Coal Co. yesterday agreed to pay a $500,000 fine after pleading guilty to criminal charges that it tampered with dust samples taken from company mines that were used to determine the risk to workers of black lung disease.

It was the second-largest criminal fine in the 14-year history of the federal Mine Safety and Health Act and the largest criminal fine ever assessed under the law for a case not involving a fatality.

Michael W. Carey, U.S. attorney for the southern district of West Virginia where the plea was entered, said Peabody "admitted to willfully violating a mandatory health standard by tampering with respirable dust samples. ... " A Labor Department official said the doctored samples led inspectors to believe that the mines were in compliance with federal standards.

In some cases, Carey said, the company took air samples outside the mine and in other instances tampered with the dust samples it took underground. The mines involved were in West Virginia, Kentucky and Illinois.

Peabody, in a statement released from its St. Louis headquarters, said the tampering had been done by employees at the mine in violation of both company and federal policy. In agreeing to pay the fine, Peabody said the company will be immune from further prosecution for any past tampering that might have occurred at its mines.

The company pledged to cooperate with the continuing federal investigation into safety practices at company mines. As part of the plea-bargain agreement Peabody said it would fire anyone who tampers with a dust sample.

Richard Trumka, president of the United Mine Workers of America, said the union was convinced "a thorough investigation into all aspects of this matter will reveal that today's conviction of Peabody Coal is just the tip of the iceberg." Trumka called on the government to take responsibility for monitoring and controlling dust in the nation's coal mines away from the companies.