Following a recipe downloaded from the Internet and using gene sequences from a mail-order supply house, researchers have assembled a manmade version of the polio virus to prove how easy it would be for terrorists to make deadly biological weapons.

Researchers at the University of New York at Stony Brook assembled the virus and then injected it into mice to show that it worked. The animals were paralyzed and then killed.

"The reason we did it was to prove that it can be done and it now is a reality," said Dr. Eckard Wimmer, leader of the biomedical research team and co-author of the study appearing Friday in the journal Science.

"This approach has been talked about, but people didn't take it seriously," Wimmer said. "Now people have to take it seriously. Progress in biomedical research has its benefits and it has its down side. There is a danger inherent to progress in sciences. This is a new reality, a new consideration."

Wimmer said the laboratory demonstration proves that eradicating a virus in the wild may not mean it is gone forever. Now, he said, biochemists can reconstruct those viruses from blueprints easily available in scientific archives and from biological supplies that can be bought through the mail.

The polio virus assembled in the laboratory is one of the simplest of the human plagues, said Jeronimo Cello, first author of the study.

"It was very easy to do," he said.

Smallpox and other lethal viruses are much more complex and difficult to assemble, but Cello said "probably in the future it would be possible."

Wimmer said it "would be very difficult now to re-create the smallpox virus, but eventually you would be able to do that."

"The world had better be prepared," he added.

Smallpox was eradicated in the wild, but laboratory specimens were retained in the United States and in the Soviet Union. Some experts worried that specimens of the virus could have been secreted away for later use as a weapon.

After last fall's terrorist and anthrax-by-mail attacks, U.S. officials became concerned about the threat of smallpox and arranged for the manufacture of enough vaccine to protect the U.S. population. They are now formulating a policy about how that vaccine should be used.

Polio is on the brink of being eradicated worldwide and there are plans to stop inoculations against the disease after it disappears from nature. Wimmer said that policy should be reconsidered. Stopping vaccination could lead to a generation of people highly susceptible to polio, enhancing its danger as a weapon.

The World Health Organization is planning to stockpile vaccines against a return of polio, and Wimmer said that policy should be followed everywhere.

"Our message is that you have to keep stockpiles of vaccines for every agent that you try to eradicate," he said.

C. J. Peters, director for the Center for Biodefense at the University of Texas Medical Center at Galveston, said experts have known for years that it was theoretically possible to assemble a virus in the lab.

"This may be the culmination of sewing Frankenstein together, but this is not the research that led up to Frankenstein," Peters said of Wimmer's work. "We've known this could be done. We've known it was just a matter of time before it was done."

Peters was critical of Wimmer's demonstration, noting "I don't think this has been very helpful.

"Wimmer is a very smart guy," he said. "He has made a lot of contributions to polio, but this is not a great contribution."

Peters said he was concerned that publicity about a synthesized virus may lead some people to believe "that there is nothing that can be done about bioterrorism -- which is not the case."

He said it is possible that viruses like Ebola could be assembled in laboratories, but there are only a few people in the world with that skill. "I don't think we need to encourage people to take this up as a hobby," said Peters.

"Bioterrorists didn't learn anything from us. Everything we did has been published before," said Wimmer. "We just put the steps together. Many laboratories could repeat what we have done."

Wimmer said gene sequences used to make the virus were ordered from supply houses that make the materials to order. He said there should be laws that tighten the distribution of such material, requiring supply houses to report any suspicious orders.

He said supply houses could quickly check genetic sequences through the computer and determine in seconds if they could be used to make dangerous virus.

"If somebody orders a number of these sequences, then the company should be required to report it," said Wimmer. "It's a simple mechanism to prevent the misuse."