Keyboards and mice.
Researchers say that the problem stems from rodents searching for food crumbs trapped between keys in empty offices at night.
To highlight the problem, the Royal Society of Chemistry team have launched a search for Britain's filthiest computer keyboard.
The organisation urged employers to adopt a clean workstation policy to ensure good hygiene standards.
Spokesman Paul Gallagher said: "We are asking staff to take shots of offensive keyboards and workstations and to email them to us.
"We will give pounds 300 to the sender of the most awful image.
"We do not need the names of the organisations and we will not publish any identification of the place or the sender."