The job action is forcing the rescheduling of thousands of surgeries and outpatient appointments at centers in Los Angeles, San Francisco, San Diego, Irvine and Davis.
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The walkout involved more than 15,000 patient care technical workers, including radiology technicians, respiratory therapists, pharmacy workers, laboratory workers and others but not nurses.
Another 24,000 other union workers, ranging from truck drivers to gardeners and cooks, were striking in sympathy, said John de los Angeles, spokesman for the American Federation of State, County and Municipal Employees Local 3299.
Emergency rooms remained open.
The hospitals said they made preparations for the strike, but it still will affect thousands of patients.
Patient care workers have been without a contract since December. Talks and mediation efforts have stalled, and the university plans to impose new employment terms next month, union spokesman de los Angeles said.
The union wants the university to stop outsourcing low-wage work that it claims is fueling widening income, racial and gender gaps for workers at UC's hospitals, clinics campuses and research facilities.
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The same issue prompted a three-day walkout by 53,000 UC workers last May, including custodians and cafeteria workers. Nurses and other medical workers walked out then in sympathy.
A University of California statement accused union leaders of spreading false information about outside service contracts. It said the number of unionized patient care workers had increased by nearly 19 percent over the past five years while outsourcing contracts had stayed relatively flat.
The employment deal to be implemented next month would grant 3-percent-a-year raises for the next 4 years for patient care and service staff, as well as offering a health plan at the same rates as other UC employees with similar salaries.
The statement from the UC president's office read in part: "As a negotiating tactic, this AFSCME-led strike is no more effective now than it was in May. Union leaders certainly have the right to express - even scream - their opinions, but the way to a deal is at the negotiating table, not on the picket lines."
The Associated Press contributed to this report.