One of the birds died during treatment at the Los Angeles Zoo and four others are still being treated there. A chick and its mother also appear to have poisoning and may undergo treatment as well.
Officials don't yet know the source of the contamination, but a U.S. Fish and Wildlife official said the birds were likely poisoned by eating the carcasses of animals that had been shot by hunters.
Lead poisoning is a known threat to the majestic birds and the main reason the state is about to ban hunting with lead bullets.
Jesse Grantham, a U.S. Fish and Wildlife condor coordinator, called the poisonings alarming and said the agency was in "crisis mode."
The California condor nearly went extinct in the 1980s, but a trapping and breeding program has helped restore the species. There are only about three dozen of the endangered birds in Southern California, and about 200 in the wild overall.
Experts believe lead poisoning is a major factor in preventing the species' recovery.
Under a ban that takes effect July 1, it will be illegal for California hunters to possess or fire lead ammunition when they are in the birds' habitat.