Doctors are excited about a genetically engineered blood-clotting agent that could let hemophiliacs treat themselves with preventive injections, the way diabetes sufferers do.

The clotting protein made by gene splicing also should end fears hemophiliacs might catch blood-borne diseases such as AIDS.About 60 percent of hemophiliacs in the United States were unknowingly infected with HIV, the AIDS virus, during the early 1980s. The virus contaminated clotting proteins derived from human blood.

New purification techniques have made blood-derived hemophilia treatments free of HIV, but some experts worry that other microbes might infiltrate this treatment again if blood remains the sole source.

Hemophiliacs have an inherited disorder that prevents them from making enough factor VIII, a protein essential for blood clotting. To stop bleeding after accidents and operations and to prevent damaging bleeding in the joints, they must take factor VIII injections.

Two companies are testing recombinant factor VIII, made through gene-splicing techniques, that should provide the treatment without possible contamination from microbes or impurities.

"We are all tremendously excited about this. It represents a tremendous advance," said Dr. Jeanne M. Lusher of Children's Hospital of Michigan in Detroit.

Lusher participated in the first large-scale testing of recombinant factor VIII.