Microsoft launched a messaging system for businesses, Microsoft Teams, on Wednesday, putting it in direct competition with fast-growing startup Slack and Facebook’s Workplace product.

The new software will be part of Microsoft’s Office 365 services, a cloud-based software suite that has more than 85 million monthly active commercial users.

Slack, whose customers include CBS cbs , BuzzFeed, universities, and the U.S. government, had over 4 million daily active users in October.

San Francisco-based Slack took out a full page ad in the New York Times on Wednesday, addressing the entry of Microsoft Teams.

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“We’re genuinely excited to have some competition,” Slack said in the ad.

Microsoft said a preview of Teams will be available to Office 365 commercial customers with enterprise or business plans, starting Wednesday.