The Code Red computer virus worm has apparently failed to attack the internet following a worldwide alert.
The worm was expected to become active at 1am (Irish time) but there was no sign it had gone to work.
However, US Government officials monitoring internet traffic say it could be a day or two before any effects of the latest attack are noticed.
Ronald L Dick, the director of the FBI's National Infrastructure Protection Centre in Washington, commented: "There has been an amazing effort to ensure that the public and private sector proofed their computers against (Code Red).
"As of now, the internet is operating normally."
According to Alan Paller, director of research at the SANS Institute, a computer security think-tank, the original Code Red worm took seven days to hit its stride.
He added: "We don't know yet whether we are safe and we won't know for sure until seven days pass with no major disruptions."
The worm can spread quickly without human intervention on unprotected Internet computer servers and threatened to slow down web traffic. It does not affect most home computers.
The malicious program can only be stopped if enough website operators install Microsoft's software patch, which plugs the security hole the worm uses to attack.
FBI officials say that more than a million people had downloaded the patch from Microsoft.