The sheriff's department for Chicago's Cook County says it's suing Craigslist, calling the popular online classifieds site the largest source of prostitution in the United States.
Sheriff Tom Dart plans a news conference later Thursday.
A call for comment before business hours to Craigslist's public relations firm was not immediately returned.
Craigslist has come under fire in recent months for the "erotic services" ads posted on its site. Several people have been arrested on prostitution charges after posting the ads online.
In November, Craigslist pledged to crack down on prostitution ads as part of an agreement with several attorneys general.