LONG BEACH — Orange County's beleaguered trauma hospital system received a major boost last week with the announcement that Long Beach Memorial Medical Center, already one of Los Angeles County's busiest trauma facilities, will assume a major role in caring for Orange County's critically injured patients.

Representatives of Orange County's health care system had been working for more than six months to enlist a fourth hospital to replace Fountain Valley Regional Hospital and Medical Center, which dropped out of the trauma network in December.

"This is a red-letter day for Orange County," Supervisor Harriett M. Wieder said at a press conference at the Long Beach hospital. "It's a perfect demonstration of the importance of regionalism in an area like emergency services. I won't talk about Orange County versus Los Angeles County; this is something that will serve the needs of both counties."

Orange County health officials said they believe the addition of Memorial will improve the level and quality of services provided to trauma patients.

The Fountain Valley hospital, for example, was not equipped to provide extensive neurological services that are needed by some critical-care patients.

The Long Beach center, founded in 1907, is recognized as a premier provider of critical-care needs, with 998 beds, a helicopter ambulance service and an emergency room that treated 1,036 trauma patients in 1989.

Gayle Bullock, Memorial's senior vice president in charge of trauma services, said the agreement will strengthen the hospital's "informal" presence in Orange County.

Bullock also said that many staff members at Memorial already have strong ties to Orange County, noting that she and the director of trauma services and chief of staff are all Orange County residents.

The hospital now serves about 60 trauma patients per year from Orange County, but officials expect an increase of 200 to 250 patients per year once the formal arrangement becomes effective.

"We are very interested in continuing to provide quality medical care to both counties, as well as fill a dire need for this important service in Orange County," Bullock said.

Orange County health officials said the addition of the Long Beach facility will relieve much of the pressure on the county's three other trauma centers but does not totally alleviate the problem of providing emergency care for growing numbers of indigent patients in the county.