On the third floor of the State Research Center for Applied Microbiology here, the sign warns: "Particularly hazardous infections."

Behind the door is a storehouse of some of the most lethal substances ever created, samples of germs and other pathogens developed for use in Soviet biological weapons. In an archive of freezers and test tubes, the third floor includes a repository for genetically engineered versions of anthrax and plague, as well as the lesser-known diseases tularemia and glanders.

The archive is one of the Cold War's most terrifying legacies. In the laboratories of Building No. 1, Soviet scientists worked in extreme secrecy for 20 years trying to build ever more deadly biological weapons, even after Moscow signed a treaty promising not to develop or stockpile them.

Now, slowly, Russia is opening the door to this and some other dark corners of the once hidden Soviet bio-weapons complex. Recently, for the first time, a sample anthrax strain was sent to the United States for analysis, the beginning of what U.S. officials hope will be a broader exchange.

As more is learned, the West is responding by pressing Moscow to tighten security to help keep bio-weapons out of the wrong hands. The United States has agreed to provide nearly $1 million for extra guards, video cameras and other protection for what had been a surprisingly lightly guarded compound.

Journalists were taken on a limited tour of Building No. 1 late last month. Here, according to Ken Alibek, a bio-weapons expert who defected to the United States in 1992, the Soviet Union carried out some of the most ambitious biological weapons research ever attempted.

As with many Soviet-era scientific facilities, it has a decaying outward appearance. Buildings are crumbling, weeds sprouting and air locks on the fourth and fifth floors looked unused. Only the third floor of Building No. 1 is now devoted to the most dangerous substances, but in earlier days, five of the nine floors were used for research on bio-weapons. From a glassed-in corridor atop the building it is possible to view the whole sprawling complex, including a 40-bed special isolation hospital built in case of accidental contamination.

In the Soviet era, Obolensk had 4,000 workers and was known as Post Box V-8724, hidden in a remote, wooded area south of Moscow. Its location was concealed, as were those of many of the most sensitive Soviet nuclear weapons facilities. It was not on any map. Today, scientists here carry out civilian biotechnology projects; they are trying to fight drug-resistant tuberculosis and preparing to manufacture high-grade insulin, which is still in short supply in Russia.

The institute opened recently for an unprecedented three-day international conference for about 200 microbiology experts from Russia, the United States and Europe. The theme of the conference was biological and ecological safety, and it was held in the same auditorium where Soviet scientists once discussed how to create the most devastating biological weapons ever conceived.

"Scientists today are the people who must create the system of resistance to biological weapons and to biological terrorism," institute director Nikolai Urakov said in an interview. Tall, with a head of white hair and a shoulders-back military posture, Urakov knows whereof he speaks. According to Alibek, Urakov, who holds the rank of general, won a Soviet prize for development of a "Q fever" weapon. Q fever is a rare disease contracted from animals that can cause pneumonia and other disorders in humans. Later, Urakov also oversaw a project, code-named "Bonfire," that involved genetically engineering new versions of such diseases as plague.

The KGB was especially interested in a new class of weapons that could damage the human nervous system and alter moods, Alibek recalled in his memoir, "Biohazard." "Victims would appear to have died of natural causes," Alibek wrote. "What intelligence service would not be interested in a product capable of killing without a trace?"

In 1979, a Soviet biological weapons accident at a top-secret military laboratory in Yekaterinburg--called Sverdlovsk in the Soviet era--is believed to have caused the world's most serious known outbreak of human inhalation anthrax; the official death toll was 66.

The bio-weapons effort is not just history. Many of the pathogen samples remain in storage, and in recent years there has been growing apprehension in the West about the possibility they could be stolen and used by terrorists. The laboratory here had just a single guard at the front door and another at the gate of the fenced compound in which it is located. It is not nearly as heavily guarded as Russian nuclear facilities, which typically have guard patrols, dogs, surveillance cameras and other perimeter controls.

The Cold War made controlling nuclear weapons a priority. Today, they are limited by treaties, and millions are spent to tighten security on nuclear materials. Likewise, the United States is helping Russia prepare to destroy some of its aging chemical weapons stocks. But biological weapons have been a far more elusive target for disarmament than nuclear weapons. There is a fine line between research to create offensive biological weapons and research to defend against them; as a result, work on offensive weapons can be concealed under the cover of research to develop vaccines.

Moreover, until the last few years, the Soviet biological weapons complex was a mystery. From defectors and other sources, it was known the weapons effort was concealed under a structure known as Biopreparat, created in 1973 to provide civilian cover for advanced research on biological weapons.

Biopreparat had laboratories and production facilities spread across the Soviet Union. Samples of the deadly pathogens that were developed are still stored in freezers and test tubes from Obolensk to Kazakhstan. (The U.S. government said it stopped its biological weapons program in 1969. The Soviet Union signed the Biological Weapons Convention in 1972 but almost immediately set about violating it.)

In 1997, representatives of the Iranian biological weapons program made an attempt to obtain technology, pathogens and expertise from the institute here and from another bio-weapons laboratory, Vector, in the Novosibirsk region, which worked on viral weapons. Andrew Weber, special adviser for threat reduction policy in the U.S. Office of the Defense Secretary, said that the Russian labs refused the Iranian overtures and that the United States "dramatically" increased their funding and began cooperating with them more closely.

Meanwhile, money from the Russian government has dwindled. Urakov said he is pleading with Moscow for about $3 million a year to keep the laboratory functioning. Along with new grants just announced, the total Western assistance to scientists, and for improved security at Obolensk, will come to about $4.5 million a year.

Earlier attempts at using diplomacy to curb the possibility of biological weapons proliferation ran into a dead end. A joint U.S.-British-Russian effort was frustrated by disagreements over mutual inspections; some Russian military microbiology labs are still off-limits.

But at Obolensk, the West is making headway, sending in scientists for cooperative research. "They are making a change--cultural, scientific and economic--and it is a huge transformation," said Randall Beatty, deputy executive director of the International Science and Technology Center, a joint U.S., European and Japanese project to help Russian weapons scientists work on civilian projects.

The project is devoting about $50 million this year to biotechnology laboratories. As dozens of U.S. scientists have come to Obolensk, and Russians to the United States, a window has opened on the true scope of the colossal Soviet biological weapons effort.

At Obolensk, the Westerners found the largest set of anthrax and tulameria samples in the world. Elsewhere, Western scientists have discovered that the Soviet Union had a separate biological weapons program--outside of Biopreparat--designed to create agents that would kill livestock and crops on a mass scale. Little is known about it, but one official said that the Soviet efforts were "weaponized," meaning that not only did they experiment with such pathogens, but tested them and developed ways of delivering them. Research institutes developed weapons that would, for example, spread foot and mouth disease or African swine fever; the testing ground was in Kazakhstan, a Soviet republic at the time.

"There is a collection of highly dangerous pathogens all over the former Soviet Union," said one official involved, "and we are just in the process of getting a handle on what's out there."