Apple accessories company DVForge announced the competition after security company Symantec claimed OS X was likely to come under increasing attack as Apple's market share in the computer market grew.
DVForge said it had so much faith in the security of OS X it did not believe a virus writer could disable the operating system.
"The contest was only cancelled because I was convinced on Saturday morning that there was some minor risk of federal law violation in continuing," the head of DVForge, Jack Campbell, wrote on the company's website. "I have been stunned by Mac users writing to us who seem to prefer to live in fear and uncertainty, with their heads down, feeling 'lucky' for the moment, rather than to actually know the truth."
Before it was pulled, virus writers were invited to infect PowerMacs connected to the internet and running the latest version of OS X, apparently without antivirus or firewall security.
The plan has been heavily criticized as a publicity stunt but the company has rejected these claims, saying it was better to settle the matter of the security of Apple's flagship operating system once and for all.
"Interestingly, this was actually a serious PR risk for us," wrote Campbell. "It is always safest to avoid controversy and to simply sit quietly on the sidelines and let the issues of the day drift by. The issue of the world at large constantly misstating the Mac OS X virus susceptibility was something we decided as a company to try and do something about."
Despite multiple attempts to contact Apple, the company did not respond for comment.
