We examined expression of the normal cellular prion protein (PrPc) in human peripheral blood mononuclear cells (PBMC) and in transfected neuroblastoma cells with a panel of six monoclonal antibodies (Mabs). While all six of the Mabs reacted strongly with the neuroblastoma cells, only four of the Mabs reacted with PrPc expressed by human PBMC. PrPc is expressed at high levels in human T cells, B cells, monocytes, and dendritic cells, but not in red blood cells. Immunoblotting studies revealed that the PrPc glycoforms and the composition of the N-linked glycans on PrPc in human PBMC are different from those of the brain or the neuroblastoma cells. In human PBMC and the neuroblastoma cell lines the N-terminal portion of the PrPc is hypersensitive to proteolytic digestion, suggesting that the N-terminus of the PrPc on the surface of a living cell lacks secondary structure. We found that the level of PrPc expressed on the surface of human T lymphocytes was up-regulated as a consequence of cellular activation. Accordingly, memory T cells express more PrPc than naïve T cells. In addition, the proliferation of human T lymphocytes stimulated with an anti-CD3 Mab was inhibited by anti-PrPc Mabs. Collectively, these results suggest that PrPc can participate in signal transduction in human T lymphocytes.