Peptide-targeted phage are internalized by choroid epithelial cells and ependyma 24 h after icv injection. Control phage (Panel A), Peptide B (Panel B), Peptide C (Panel C) or Peptide D (Panel D) -targeted phage (1 × 1011/10 ul) were injected into the lateral ventricle of anesthetized rats and 24 h later brain were collected and frozen sections immunostained with anti-M13 antibodies. Whereas low background immunoreactivity was observed in choroid plexus epithelial cells after control (untargeted) phage injections (Panel A), Peptide B (Panel B), Peptide C (Panel C) and Peptide D (Panel D) -targeted phage injections resulted in strong M13 immunoreactivity was observed within most of the epithelial cells of the choroid plexus. Similarly, very low immunoreactivity was observed in the ependymal cells after a wild type phage injection (Panel E) but after Peptide B (Panel F), Peptide C (Panel G) and Peptide D (Panel H) targeting, ependymal cells of the lateral ventricle clearly show intracellular immunostaining. Notice the lack of M13 immunoreactivity observed in the brain parenchyma in all treatments.