Researchers have developed a new law-enforcement technology called DNA fingerprinting in which a small sample of blood or hair can be used to identify the person it came from with virtual certainty, according to officials of two companies that plan to market the technology in this country.

The technique could become one of the most important tools in some types of criminal cases because it will enable investigators to link physical evidence quickly and precisely to a single person, they said. It will make identifications possible in a large number of criminal and paternity cases where fingerprinting is inadequate or impossible.

Because the technique is so effective, police will build DNA identification files like the massive fingerprint files that now exist, according to James E. Starrs, a lawyer and forensic expert at George Washington University. He is an adviser to Cellmark Diagnostics, a British company that last week announced opening of a laboratory in Germantown, Md.

Because the DNA, or dioxyribonucleic acid, of every human is unique it is possible to match a sample of DNA taken from a blood stain or semen sample with a DNA sample from a suspect. A child's DNA, which is inherited in identifiable patterns from each parent, can be compared with that of a possible father's to establish paternity.

In one recent case in Britain, police had two rape-murders committed three years apart. They suspected they were committed by the same man but could not prove it.

Using DNA technology to compare semen samples from the cases, they found the crimes were committed by the same man. But their key suspect was not him, and he was freed when his DNA did not match that taken from semen in the cases, according to Cellmark officials.

In a recent case in Oklahoma, a bloodstain on a vacuum cleaner was subjected to DNA analysis, allowing the police to say with confidence that a missing man was at the scene of the crime, according to John K. Winkler, president of Lifecodes Corp., a Westchester County, N.Y., firm that started DNA analysis this year.

Cellmark officials said last week that DNA analysis can be done with samples as small as one or two drops of blood, a few hair roots or a trace of semen.

They said the technique could be useful in a number of the 50,000 paternity cases, 80,000 rapes and 20,000 homicides logged annually in the United States.

"In rape cases in which sperm can be recovered, there is just no way that could be analyzed until now," Lifecodes' Winkler said. "Now we can easily get a perfect match."

More often, he said, suspects can be eliminated. "One of the best things with DNA analysis is that there are no false positives. If it's not the guy, it just isn't. The DNA just won't match at all."

In other criminal cases, bloodstains can be identified only crudely, by blood groups shared by a large number of people. The DNA technique can match blood to suspects with near certainty.

The techniques work by taking any sample that has cells and extracting the DNA. Using enzymes that snip DNA at precise points, the DNA is cut into fragments and lined up according to size. Because DNA is different in all people except identical twins, the pattern that the varying lengths of DNA make will always be the same for one person, and always different from another.

Once the DNA from the sample is sorted into a pattern, then a sample from the suspect is put through the same process. The resulting patterns appear like bar codes used to price and identify packaged goods and can easily be matched.

The techniques are beginning to be used as investigative tools and have occasionally been admitted in court cases. But, according to Starrs, there are at least two major obstacles to their regular use in U.S. courts.

First, in order to have scientific evidence regularly accepted in court, it must pass a test of reliability that says, in effect, that the techniques are accepted by the whole scientific community.

It also not clear, Starrs said, under what circumstances suspects can be forced to give police a sample of hair or blood to be used in the DNA identification.