Sulfated polyanions, including pentosan polysulfate (PPS) and heparan mimetics, number among the most effective drugs that have been used in experimental models of prion disease and are presumed to act in competition with endogenous heparan sulfate proteoglycans as co-receptors for prion protein (PrP) on the cell surface. PPS has been shown to prolong the survival of animals after intracerebral perfusion and is in limited use for the experimental treatment of human transmissible spongiform encephalopathies (TSEs). Here, PPS is compared with CR36, a new heparan mimetic. Ex vivo, CR36 was more efficient than PPS in reducing PrPres in scrapie-infected cell cultures and showed long-lasting activity. In vivo, CR36 showed none of the acute toxicity observed with PPS and reduced PrPres accumulation in spleens, but had only a marginal effect on the survival time of mice infected with bovine spongiform encephalopathy. In contrast, mice treated with PPS that survived the initial toxic mortality had no detectable PrPres in the spleens and lived 185 days longer than controls (+55%). These results show, once again, that anti-TSE drugs cannot be encouraged for human therapeutic trials solely on the basis of in vitro or ex vivo observations, but must first be subjected to in vivo animal studies.