The unionized nurses, social workers and therapists are protesting efforts to reduce their health care coverage and retirement benefits.
They are also protesting what they call chronic short staffing at some of the Kaiser health care facilities.
About 2,500 Kaiser employees are on strike in Los Angeles. The California Nurses Association - the union organizing the strike - estimates that 23,000 nurses will walk off the job statewide.
Kaiser says it continues to bargain in good faith and will try to minimize the impact of the strike on patients.
The Associated Press contributed to this report.