Transgenic mice expressing bovine prion protein (PrP)(C) (Tgbov XV mice) display remarkably shorter incubation times for cattle-derived bovine spongiform encephalopathy (BSE) infectivity than do nontransgenic mice. To verify that this phenomenon reflects increased sensitivity, we challenged Tgbov XV mice and conventional RIII mice with a BSE brain-stem homogenate of known infectivity titer in cattle. An end-point titration experiment in Tgbov XV mice revealed their superior sensitivity, which exceeded that of RIII mice by at least 10,000-fold and even that of cattle by approximately10-fold. Moreover, Tgbov XV mice were challenged with various tissues from cattle with end-stage clinical BSE, and infectivity was found only in the central and peripheral nervous system and not in lymphatic tissues; the only exception was the Peyer's patches of the distal ileum, which most likely are the site of entry for BSE infectivity. These results provide further indication that the pathogenesis of BSE in cattle is fundamentally different from that in sheep and mice, due to an exclusive intraneuronal spread of infectivity from the gut to the central nervous system.