Doctors in California have performed liver transplants on two patients who mistakenly picked and ate lethal Death Cap mushrooms during a New Year's outing in Marin County.

The second operation, on 45-year-old Milhelm Winter, was performed Saturday night.

Winter and his 19-year-old companion, Cynthia Zheng, are believed to be the second and third persons in the United States to receive liver transplants to treat life-threatening cases of mushroom poisoning.

"They're both critical but stable," a University of California at Los Angeles nursing supervisor said. "You can't tell anything at this stage. It's too early to tell."

Zheng, who became ill first from eating the deadly mushrooms, received a new liver Wednesday in an operation that took seven hours. She and Winter had relied on a field book containing pictures of dangerous mushrooms when they picked foods for an outdoor meal, according to Dr. Robert Benjamin of the Alameda County Health Care Services Agency.

However, mushrooms can be difficult to identify, even when a book is used, he said.

The mushrooms they ate were found to be Amanita phalloides, one of a group of mushrooms containing a toxin that kills liver and kidney cells.