BOSTON, FEB. 13 -- The death rate from a bacterial infection that kills between 30,000 and 100,000 Americans each year can be cut by nearly 40 percent with a new drug that consists of a factory-made antibody that binds to the bacteria's deadly toxin, rendering it much less harmful, according to a new study.

The research, reported in Thursday's New England Journal of Medicine, found that while the experimental treatment saves lives in this medical emergency, it is far from foolproof. Even with the therapy, about 30 percent of the patients died. Without the new treatment, the death rate is 49 percent.

The new approach is intended to control a form of blood poisoning called Gram-negative bacteremia. The disease occurs when ordinarily harmless bacteria, belonging to a class called Gram-negative, invade the bloodstream, often when people already are weak from other health problems. The infection causes life-threatening complications, including low blood pressure, fever and kidney failure.

The disease results from the body's reaction to a toxic substance released into the blood by the bacteria. In the latest approach, researchers manufactured large amounts of an antibody that is a copy of the antibody the infected person's immune system would make with a shape tailored to bind to the toxin and neutralize it. Antibodies of any desired shape can be manufactured in the laboratory using cultures of specially modified antibody-making cells from the immune system.

Doctors tested the manufactured antibody -- known as HA-1A or Centoxin -- on 543 people at 24 hospitals who were suspected of having Gram-negative bacteremia. The study was directed by Elizabeth J. Ziegler, a physician at the University of California at San Diego and sponsored by Centocor Inc. of Malvern, Pa., which makes the antibody.

The patients were randomly assigned to receive the antibody or a placebo, and 200 of them eventually turned out to have had Gram-negative bacteremia.

"Our results indicate that HA-1A is safe and that it substantially reduces mortality in patients with sepsis {infection} and Gram-negative bacteremia," the researchers wrote.

"Unfortunately," said Seldon M. Wolff of the New England Medical Center, "a large number of patients still died despite antibody therapy. Optimal therapy of this condition in the future will require additional agents."