Scientists using a powerful X-ray technique have taken the first snapshot of the contact made by the AIDS virus as it snares its two target sites on the surface of a human cell. The pictures show the intricacy of the virus' methods for dodging the human immune system and confirm some researchers' view that the AIDS vaccine recently approved for testing is unlikely to work.

At the same time, the sight of the virus' defenses brings to light certain weak points against which new drugs and perhaps vaccines could be designed.The visualization of the virus' attack, a computer-generated image based on X-ray data, is the fruit of eight years of work by researchers at Columbia University and the Dana-Farber Cancer Institute in Boston.

It depended on a technique powered by X-rays from the particle accelerator at the Brookhaven National Laboratory on Long Island. The results are being published in articles in this week's issues of Nature and Science.

The new work is important because it captures the critical moment of infection when one of the probes that stud the surface of the virus makes contact with the outer membrane of a target cell.

Although researchers understood the process of infection in a general way, they can now see what happens with the precision of an atom's eye view.

Dr. Joseph Sodroski, one of the report's authors, said, "You can immediately visualize the conserved elements of the protein, which make attractive targets for drugs or vaccines," referring to the regions of the probe that the virus does not change by mutation.