The union, representing pharmacists, nursing assistants and other health care workers, say the strike is over low staffing levels. However, UC officials say the issue is a reduced benefit pension offer.
Nurses are not striking and emergency rooms remain open.
Elective surgeries have been postponed and the hospitals have hired temporary workers.
Health care workers from hospitals at UCLA, UC Irvine, UC San Diego, UC Davis, and UC San Francisco are participating in the strike.