LOS ANGELES (AP) _ The American Red Cross canceled bloodmobile trips and nurses walked picket lines today in the first strike in the 40-year history of the regional blood services unit.

The 225 nurses in Los Angeles and Orange counties walked off the job Sunday, when the Red Cross canceled 11 scheduled bloodmobile trips, mostly to churches. Seven blood donor centers were closed and 21 bloodmobiles were idled today, Red Cross spokeswoman Gerry Sohle said.

''We can't run them without nurses,'' said Ms. Sohle. Most donations come from mobile units, but the spokeswoman said there was little immediate danger that blood supplies would run low.

''We decided to cancel the bloodmobiles for the safety of donors,'' said agency spokesman Ralph Wright on Sunday, adding that it could be several days before the service is fully restored.

The dispute between Local 535 of the Service Employees International Union and the Red Cross centers on staffing, unscheduled reassignments and management refusal to pay licensed vocational nurses as much as registered nurses, said union spokeswoman Theresa Conrow.

About 100 chanting strikers carrying bright yellow ''ON STRIKE'' signs picketed Sunday outside the Los Angeles Red Cross Blood Services Center, and about 30 more walked the line at a Van Nuys center.

Union leaders had said a strike would seriously affect blood supplies.

''It's just not true'' that adminstrative personnel can substitute for regular staffers, said Ligaya Fontanilla, head nurse at one blood center. ''They haven't handled needles in years,'' she said.

But Wright said supplies were well above optimum. The agency has 17,000 units of blood on hand, he said.

''We have a lot of blood. As a matter of fact, we've never had so much blood,'' said Ms. Sohle on Sunday. ''If we didn't collect another drop, we could go through the rest of this week. Patients are not in danger.''

The striking nurses want donations to continue, and will provide lists of hospitals and other donor centers, Ms. Conrow said.

Ms. Sohle said two Red Cross blood donor centers in Los Angeles and one in Orange County would be open today using supervisors.

The contract expired Tuesday but a walkout was postponed until a vote could be taken. Union members rejected the latest contract offer Saturday by a 2-1 margin.

The union, which represents registered and licensed vocational nurses, sought a 6 percent raise for the first year of a three-year contract, followed by slightly lesser pay hikes. The Red Cross offered 4 percent, plus an additional 5 percent for vocational nurses for the first year, Ms. Conrow said. Top monthly pay for a Red Cross registered nurse is now about $2,041.

Union negotiators also want contract language guaranteeing a minimum staffing ratio of nurses to donors. They said donor safety could be compromised because the ratio is rising and donor screening takes far longer because of new safeguards to avoid the spread of AIDS.