We screened six mouse monoclonal antibodies (mAbs) against prion protein (PrP), which were previously established in our laboratory, for inhibitory activity against PrPSc-accumulation in scrapie-infected cell lines and identified two mAbs, 3S9 and 2H9, as possessing this inhibitory activity. mAb 3S9 recognized an epitope including 154th tyrosine in the helix I region of PrP, while mAb 2H9 recognized a discontinuous region that included helix 1. In three scrapie-infected cell lines infected with different mouse-adapted scrapie strains, mAb 3S9 strongly inhibited accumulation of PrPSc, while mAb 2H9 moderately inhibited accumulation of PrPSc, indicating that inhibition of prion propagation by mAbs may be dependent on PrPSc characteristics. Furthermore, mAb 3S9 completely excluded PrPSc from these cell lines. These results suggest that mAbs 3S9 and 2H9 might be useful for clarifying the mechanisms of prion propagation and prevention by PrP-specific antibodies, and for tracing the conversion of PrPC to other PrPSc isoforms. (c) 2005 Elsevier Inc. All rights reserved.