SAN DIEGO -- Robert F. Kennedy Jr. canceled his speech Thursday at a University of California, San Diego pre-graduation celebration, saying he would not cross the picket lines of University of California hospital workers, university officials announced.

UCSD Chancellor Marye Anne Fox will likely take the environmental activist and author's place at the "All Campus Graduation Celebration," said UCSD spokeswoman Pat JaCoby.

Commencement ceremonies at UCSD are slated to begin on Saturday.

The American Federation of State, County and Municipal Employees Local 3299 represents 20,000 UC nurses, medical technicians, respiratory therapists, custodians, cafeteria workers and security officers, who have been negotiating a new contract since August 2007. They are seeking higher wages.