Governor Arnold Schwarzenegger has signed legislation making it illegal to smoke in a vehicle carrying children.
The bill by state Sen. Jenny Oropeza, D-Long Beach, authorizes fines of up to $100 per violation.
The measure adds to a growing list of smoking prohibitions in California, which include smoking in indoor workplaces and within 25 feet of a playground.
Oropeza says her bill will reduce children's exposure to second-hand tobacco smoke.
A Harvard School of Public Health study issued last year found that second-hand smoke in a car can be up to 10 times more dangerous than in a home.
The ban takes effect Jan. 1.