MANY viruses have evolved mechanisms to avoid detection by the host immune system, Herpes simplex virus (HSV) expresses an immediate early protein, ICP47, which blocks presentation of viral peptides to MHC class I-restricted cells(1). The properties of the newly synthesized class I molecules in HSV-infected cells resemble those of cell lines deficient in the transporter associated with antigen processing (TAP) in that class I molecules are retained in the endoplasmic reticulum(1,2), and the heavy chain and beta(2)-microglubulin subunits dissociate in detergent extracts but the complex can be stabilized by peptides(1). We show here that ICP47 binds to TAP and prevents peptide translocation into the endoplasmic reticulum.