Chronic wasting disease (CWD), a prion disease affecting free-ranging and captive cervids (deer and elk), is widespread in the United States and parts of Canada. The large cervid population, the popularity of venison consumption, and the apparent spread of the CWD epidemic are likely resulting in increased human exposure to CWD in the United States. Whether CWD is transmissible to humans, as has been shown for bovine spongiform encephalopathy (the prion disease of cattle), is unknown. We generated transgenic mice expressing the elk or human prion protein (PrP) in a PrP-null background. After intracerebral inoculation with elk CWD prion, two lines of "humanized" transgenic mice that are susceptible to human prions failed to develop the hallmarks of prion diseases after >657 and >756 d, respectively, whereas the "cervidized" transgenic mice became infected after 118-142 d. These data indicate that there is a substantial species barrier for transmission of elk CWD to humans. Copyright © 2005 Society for Neuroscience.