Three U.S.-based scientists won the 2008 Nobel Prize in Chemistry yesterday for their development of a green fluorescent protein from jellyfish that has provided researchers with their first new window into the workings of the cell since development of the microscope.

Roger Y. Tsien, 56, of the University of California San Diego, along with Martin Chalfie, 61, of Columbia University and Osamu Shimomura, 80, a Japanese-born researcher who works at the Marine Biological Laboratory in Woods Hole, Mass., will share the $1.4 million prize for developing the protein that the Nobel committee called "a guiding star for biochemists, biologists, medical scientists and other researchers."

The protein can be attached to any of the 10,000 individual molecules within a living cell, allowing researchers for the first time to trace their paths as they wind through the complex pathways of life.

It is "an essential piece of the scientific toolbox," said Jeremy M. Berg, director of the National Institute of General Medical Sciences, which has funded work by all three of the prize winners.

In a news conference yesterday, Dr. Chalfie said he had slept through phone calls from Sweden and did not know about the prize until he woke up and checked his laptop. "It's not something out of the blue, but you never know when it's going to come, or if it's going to come."

Dr. Shimomura told the Japanese broadcaster NHK that he was surprised to receive the chemistry Nobel "because I was rumored as a potential candidate for the Nobel Prize in physiology or medicine."

In a phone news conference, Dr. Tsien said he felt "a bit like a deer caught in the headlights. ... Fundamentally, I'm no smarter today than I was yesterday."