The method should help tell which laboratory's stock of anthrax is closest to that used by the attacker. That could narrow the search to people with access to that particular laboratory and its stock of anthrax.

The researcher, Dr. Paul Keim of Northern Arizona University, made the announcement here at a national conference on microbial genomes.

Asked which laboratories had provided the stocks he studied, Dr. Keim said his agreement with the Federal Bureau of Investigation prevented him from discussing the case. The F.B.I., which refused to comment on his remarks, has said it has no firm suspects, but it has focused on insiders who may have had access to anthrax in laboratories.

Many strains of anthrax exist, but the one used in the attacks is called Ames, first isolated from a cow in Texas in 1981. Because of the strain's virulence, it was studied for years by the Army's laboratory for biological warfare defense at Fort Detrick, Md., and was distributed to several laboratories in the United States and abroad to help them test vaccines.

After the anthrax attacks, researchers tried to discriminate between the various stocks of Ames to see whether they could pinpoint the laboratory of origin. But since all the stocks came from a single source, the bacteria were essentially members of a single large clone, as alike genetically as identical twins.

A DNA fingerprinting test for anthrax bacteria, similar to the test used on humans in criminal cases, had been developed by Dr. Keim and colleagues.

Dr. Keim's fingerprinting test, which was based on eight points of difference, could not distinguish between the different stocks of Ames anthrax, and he set about trying to develop more markers, which are sites on the DNA at which some anthrax bacteria have a different sequence of DNA letters from other bacteria.

To help in the search for new markers, the National Science Foundation asked the Institute for Genomic Research in Rockville, Md., to decode the full DNA sequence of the anthrax bacteria recovered from Robert Stevens, the photo editor who died in the Florida attack. The institute was already sequencing the full genome of the Ames strain owned by the Fort Detrick laboratory, so it would be in a position to look for DNA differences throughout the bacterium's genome.

The institute focused on the main chromosome of the bacteria, a large ring of DNA now known to contain 5,167,515 DNA letters holding information for 5,960 genes. The bacterium also contains two small rings of DNA known as plasmids, which carry the genes essential for its virulence. The plasmid's DNA was decoded several years ago by scientists at the Los Alamos Nuclear Laboratory.

Dr. Keim's success came from studying a site on the second of these plasmids called a poly-A tract. He found that Ames stocks held in different laboratories varied in the number of A's -- one of the four units of DNA -- they contained in the poly-A tract. The number of A's varied from 8 to 25, the exact number depending on the laboratory that provided the stock.

On the basis of the poly-A test, he said, he has been able to distinguish between the Ames strains of anthrax held in four laboratories, and in a natural Ames isolate taken from a goat in 1997. Because of his agreement with the F.B.I., Dr. Keim would not name the laboratories or say from how many other laboratories he had received samples.