The lawsuits accuse the defendants collectively of flooding Microsoft's computer systems and its customers with more than 2 billion deceptive unsolicited e-mail messages.
The 15 lawsuits address some of the most misleading, deceptive and offensive spam e-mail received by Microsoft customers, the company said in a news release.
Microsoft cited Washington's strong anti-spam law, which allows Internet service providers to take action against spammers to protect consumers.
"We need an aggressive, sustained and comprehensive assault by industry, government and consumers to stop spam," said Washington Attorney General Christine Gregoire, who joined company officials at a news conference near Microsoft's Redmond headquarters. "Today's lawsuits are exactly the kinds of action we need to put illegal spammers out of business."
Unwanted computer e-mail is the top consumer complaint in her office, Gregoire said.
In London, meanwhile, Jean-Phillipe Courtois, senior vice president and CEO of Microsoft Europe, Middle East and Africa, announced a regional anti-spam initiative that includes two lawsuits alleging the unlawful gathering of e-mail account names and other illegal spamming practices, in violation of United Kingdom law.