The National Union of Healthcare Workers say employees are upset over proposed cuts to health care and retirement benefits.
"We want to send a message clear and loud that we're here for patient care, we're here for unfair labor practice and we hope that Kaiser gets the message because we're not going to back down," said Elizabeth White, a Kaiser Permanente employee.
It's their fourth walkout since contract negotiations started in 2010.
Kaiser officials say the union has failed to respond to their proposals on wages and benefits.
"We've been at the negotiating table for about two years. We've negotiated close to 100 tentative agreements. We've had an economic proposal since mid-last year and have yet to receive a meaningful response," said Mark Costa, executive director of Kaiser Permanente LA Medical Center.
All Kaiser hospitals and medical offices plan to stay open during the strike. Kaiser has hired replacement workers and says patient care will not be compromised.
The striking workers are expected to be back at work on Wednesday.
The Associated Press contributed to this report.