he European Patent Office revoked a patent yesterday held by Roche on an enzyme crucial to one of the biotechnology industry's fundamental processes, executives said. The enzyme, known as Taq DNA polymerase, is used in the polymerase chain reaction, a technique to amplify minuscule amounts of DNA for use in medical diagnosis and DNA fingerprinting.

The decision is the latest victory in a long legal battle by Promega, a small company in Madison, Wis., that, along with several other companies, challenged the European patent. Promega sells the enzyme, which was first isolated from microbes in the hot springs at Yellowstone National Park. It says Roche has demanded inordinate royalties.

In 1999, the American patent was overturned by a federal court, which ruled that it had been obtained fraudulently. That decision is on appeal.

Melinda Griffith, general counsel of Roche Molecular Systems, said the European decision would have little effect because the revoked patent covered only the natural form of the enzyme while most users had shifted to new forms made through genetic engineering. Sales of enzymes for the polymerase chain reaction totaled about $100 million, she said, adding that more than 25 companies had agreed to pay royalties.