Nearly 5,000 nurses in 15 Northern California hospitals began a two-day walkout Wednesday to protest what they said were inadequate contract offerings. Officials with the Sutter Health hospital chain said replacement workers are in place and that no disruptions in service were expected. The nurses have been in negotiations since the spring with many of the not-for-profit hospitals in the San Francisco Bay area and the Sacramento area. The California Nurses Association union is protesting what it says are unacceptable proposals on health care and retirement benefits and staffing levels.

"It's come to this because Sutter has refused to listen to the concerns of their nurses," said Charles Idelson, the union's communications director. Many nurses "can't even take breaks a significant percentage of the time because of inadequate staffing," he said.

Liz Jacobs, a registered nurse who is spokeswoman for the union, said that in most of the Sutter hospitals' proposals, new health care plans would cost workers more and give them fewer options. The union has also objected to what it sees as inadequate health care benefits for its retirees.

But spokeswomen for four hospitals in the chain said their contract offerings would not scale back health or retirement benefits.

Jonnie Banks, a spokeswoman for Eden Medical Center, which has campuses in Castro Valley and San Leandro, said contract proposals there included an 18 percent wage increase over four years and no reduction in current health care packages or retirement benefits.

Aside from the dozens of people picketing outside several hospitals, Sutter officials said, the strike will not be felt by patients.

"Babies are being born, surgeries are being done," said Carolyn Kemp, an Alta Bates spokeswoman. "I have more nurses today than I need."