That is what employees at Weyco, an insurance-benefits administrator in this small central Michigan town, found out.

Under a new policy that legal specialists say is the first of its kind, Weyco began testing its 200 employees for smoking in January. And the company put workers on alert: In the future, they will be subject to random testing. If they fail, they're fired.

Rather than take the mandatory Breathalyzer test, four employees left the company. And Weyco's strict no-smoking policy is within the bounds of employment law in Michigan. The state is one of 20 that has no laws preventing employers from firing workers who smoke even when they are not at work.

''What's next?'' asked Kary L. Moss, executive director of the American Civil Liberties Union of Michigan, speculating on other behavior that could cost workers their jobs. ''Sitting in the sun? Getting pregnant?''

In fact, employers in 46 states have significant legal leeway to tell workers what they can and cannot do once they leave the office. As a result, companies have done more than tell workers not to smoke. Until the mid-1990s, the airlines enforced policies that limited how much a flight attendant could weigh. In the 1980s, Electronic Data Systems, the computer software company founded by Ross Perot, had a policy barring facial hair and fired an employee who said that he wore a beard for religious reasons.

In 1989, a company in Indiana fired an employee for drinking after work, a violation of the company's no-alcohol policy. And in September, a company in Alabama fired a woman who drove to work with a Kerry-Edwards bumper sticker.

But firings for behavior away from work have been isolated. Legal specialists say that no company has ever gone as far as Weyco.

Employers have targeted smokers for years. Since the mid-1980s, Alaska Airlines has refused to hire smokers and tells job applicants that they will be tested for nicotine use. In 2004, Union Pacific decided to stop hiring smokers and now asks applicants to disclose whether or not they smoke. But as long as employees have said they do not smoke, that has been proof enough.