Creutzfeldt-Jakob disease (CJD) is a transmissible neurodegenerative disease of humans caused by an unidentified infectious agent, the prion. To determine whether there was an involvement of the host-encoded prion protein (PrPc) in CJD development and prion propagation, mice heterozygous (PrP+/-) or homozygous (PrP-/-) for a disrupted PrP gene were established and inoculated with the mouse-adapted CJD agent. In keeping with findings of previous studies using other lines of PrP-less mice inoculated with scrapie agents, no PrP-/- mice showed any sign of the disease for 460 days after inoculation, while all of the PrP+/- and control PrP+/+ mice developed CJD-like symptoms and died. The incubation period for PrP+/- mice, 259 +/- 27 days, was much longer than that for PrP+/+ mice, 138 +/- 12 days. Propagation of the prion was barely detectable in the brains of PrP-/- mice and was estimated to be at a level at least 4 orders of magnitude lower than that in PrP+/+ mice. These findings indicate that PrPc is necessary for both the development of the disease and propagation of the prion in the inoculated mice. The proteinase-resistant PrP (PrPres) was undetectable in the brain tissues of the inoculated PrP-/- mice, while it accumulated in the affected brains of PrP+/+ and PrP+/- mice. Interestingly, the maximum level of PrPres in the brains of PrP+/- mice was about half of the level in the similarly affected brains of PrP+/+ mice, indicating that PrPres accumulation is restricted by the level of PrPc.