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As a result of the deal, WorldCom will offer ISS's managed intrusion detection, remote vulnerability scanning, antivirus and emergency response and forensics services.
The new services will complement the managed firewall, virtual private network and authentication services that WorldCom already offers.
WorldCom struck the deal with ISS because customers wanted an integrated set of offerings from a single vendor.
Companies with offices spread across the globe are likely to be the first customers for WorldCom's new services.
ISS operates its managed services from six security operations centres around the world, allowing it to serve global customers 24 hours a day.
The services are available immediately and range in price from a few hundred dollars a month for vulnerability scanning to $2,300 (£1,574) a month for fully managed intrusion detection.