POUGHKEEPSIE, N.Y., SEPT. 5 -- IBM unveiled a new generation of mainframe computers today that it said would double the power of its most profitable machines and cement their position at the center of office computing.

The announcement, made at the company's U.S. mainframe factory here, was billed as the most important rollout for International Business Machines Corp. in 25 years.

The success of the new machines is crucial to the world's largest computer company because it derives about half of its income from mainframes and associated equipment and software.

The move was well-publicized in advance, but industry experts said they were still impressed with the scope of the company's new strategy.

The announcement crystallized a new vision for how IBM believes mainframe computers should fit into today's decentralized computer systems, where desktop machines are almost as common as telephones.

"You saw it piecemeal before," said industry consultant Sam Albert. The new machines show that IBM intends the mainframe to be the "traffic cop" of computer networks, said Albert, a former IBM executive.

In addition to announcing 18 new machines, IBM said it would provide systems that will allow mainframes to manage information on all sorts of computers throughout a company.

Such systems would address the growing concern of companies that too much information is spread among desktop computers that don't communicate well with each other.

The company said it also will provide products that will make it easier to connect non-IBM computers to a network of IBM machines.

IBM's new mainframes provide about twice the processing power of the three mainframe lines they replace and are priced from $70,480 to $22.8 million. They are IBM's response to recently announced high-end models from Japanese manufacturers.

On Tuesday, Fujitsu Ltd. announced a new mainframe it said would be the world's fastest. IBM dismissed that claim, saying it has provided the most powerful machines for years and will continue to do so.

The new computers are part of a model line IBM calls System-390. It is intended to be the successor to IBM's original mainframe line, the System-360, which was introduced in 1964, and the System-370, introduced in 1970.

Among the machines' advances are fiber-optic lines that, compared with the copper lines they replace, more than double the rate at which information can be transferred to the computers from the huge data-storage devices used with mainframes.

In addition, IBM announced the first encryption device that can be built into a computer. The optional coding device would be useful to protect the security of transactions such as electronic funds transfers between banks, the company said.

The stock market reaction was muted. IBM shares finished off 12 1/2 cents at $102.37 1/2.