If substantial amounts of polonium 210 were used to poison Alexander V. Litvinenko, whoever did it presumably had access to a high-level nuclear laboratory and put himself at some risk carrying out the assassination, experts said yesterday.

Polonium 210 is highly radioactive and very toxic. By weight, it is about 250 million times as toxic as cyanide, so a particle smaller than a dust mote could be fatal. It would also, presumably, be too small to taste.

There is no antidote, and handling it in a laboratory requires special equipment. But to be fatal it must be swallowed, breathed in or injected; the alpha particles it produces cannot penetrate the skin. So it could theoretically be carried safely in a glass vial or paper envelope and sprinkled into food or drink by a killer willing to take the chance that he did not accidentally breathe it in or swallow it.

“This is wild,” said Dr. F. Lee Cantrell, a toxicologist and director of the San Diego division of the California Poison Control System. “To my knowledge, it’s never been employed as a poison before. And it’s such an obscure thing. It’s not easy to get. That’s going to be something like the K.G.B. would have in some secret facility or something.”