Gabe Newell, managing director and co-founder of Valve, told VideoGamer.com at the Leipzig Games Convention that they will release a Xbox 360 and PC demo of Left 4 Dead.
"We will be releasing demos for both the Xbox and for the PC. I don't know what the date is for release on that, though. I think it's going to contain the first part of one of the campaigns. I think it'll probably be Hospital but I'm not sure. That's a decision that Doug Lombardi is making." Newell confirmed that you'll be able to play single-player and co-op in the demo.
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