A bill that would require most dogs and cats in California to be spayed or neutered has brought howls of protest from breeders and threats from the American Kennel Club to pull the United States' second-largest dog show from the state.

The measure — which would be the most sweeping statewide pet-sterilization law in the country — passed the state Assembly by a single vote earlier this month and goes next to the Senate, where its prospects are uncertain despite the support of animal-loving celebrities such as Pamela Anderson, Lionel Richie and retired "Price Is Right" game-show host Bob Barker.

The law is aimed at reducing the estimated 500,000 unwanted dogs and cats destroyed in California animal shelters each year.

"The more animals neutered and spayed, the fewer animals born, the fewer animals coming into our shelters, and fewer animals are euthanized," said Pat Claerbout, president of the California Animal Control Directors Association.

The bill would require pet owners to sterilize their dogs and cats by the time they are 4 months old, or face a $500 fine.

Licensed breeders of purebreds would be exempt. But the law does not spell out which dog and cat breeds would be covered, and breeders wanting an exemption would have to apply for one from their local animal control authorities. The bill leaves it up to counties and cities to set the price.

Professional breeders complain that the measure would amount to social engineering for animals.

"I think what's happening in California is socialism in its prime," said Janet Wahl, who breeds Havanese and Yorkshire terriers. She said she fears the government will put itself in the position of deciding which dog breeds can reproduce and which cannot.

The American Kennel Club, which every December sponsors the Eukanuba National Championship, the nation's largest dog show after New York's Westminster Kennel Club event, has threatened to pull the internationally televised competition out of California if the bill succeeds.

Last year's show drew 28,000 people and was credited with injecting $21.7 million into the local economy.

"Should this bill become law, I fear it could be a catalyst for other states. As the old saying goes, `As California goes, so goes the nation,"' AKC chairman Ron Menaker said in a letter to members.

At least 25 states require that dogs and cats adopted from shelters be sterilized.

Governor Arnold Schwarzenegger has not taken a position on the California bill. He and his wife, Maria Shriver, own two male dogs.

Schwarzenegger press secretary Aaron McLear declined to say whether the pets were neutered — "out of respect" for the dogs.