HELENA, Mont. (AP) _ The National Park Service agreed Thursday to drop plans to shoot bison in Yellowstone National Park this spring as part of a livestock disease research project.

The Fund for Animals announced the agreement, calling it a major victory that will save 22 bison from execution.

Federal rangers shot three pregnant bison Monday for a study of whether brucellosis, an infectious disease that causes domestic livestock to abort their young, can be transmitted from bison.

The project called for killing 25 bison to obtain tissue samples for laboratory tests.

The killings were delayed when U.S. District Judge George Revercomb in Washington granted the animal-rights group a hearing on its request for a temporary restraining order.

Park spokeswoman Joan Anzelmo said because of the court action and Monday's public opening of the park, officials decided they had missed their best opportunity to kill pregnant bison and obtain tissue samples.

Don Davis, a veterinary pathologist and Yellowstone project leader, said the project will go forward eventually.

''We will continue it, jumping through the right hoops and whatnot, probably next fall or next winter at this time,'' he said in a telephone interview from Bryan, Texas.

Brucellosis infects large numbers of the estimated 3,000 bison in Yellowstone. Earlier testing found that 54 percent of the park's northern herd had at least been exposed to the brucellosis bacteria. But park officials said the lab tests were essential to determining how many animals actually carried the disease and were infectious.

Montana ranchers contend the diseased bison are a threat to their livestock industry and the animals have been killed when they leave the park in winter and approach Montana cattle pastures.

Since 1985, more than 700 bison have been killed by Montana hunters and state game wardens. Yellowstone is in the northwest corner of Wyoming, abutting Montana.

On Tuesday, Gov. Stan Stephens signed a measure halting civilian hunting of bison. Game wardens and Yellowstone rangers will still be authorized to kill wandering animals.

Wayne Pacelle, national director of the Fund for Animals, said the final agreement allowed the fund to dismiss its lawsuit.

''If a human is suspected of having brucellosis (undulant fever), we don't shoot the person to test for the disease. Similarly, it is anachronistic and cruel for researchers to kill bison to answer questions about brucellosis infection,'' Pacelle said.

Anzelmo said future plans would consider public concern, visitor safety, alternative methods of gathering data and ''possible legal challenges.''