Virally infected cells degrade intracellular viral proteins proteolytically and present the resulting peptides in association with major histocompatibility complex (MHC) class I molecules to CD8+ cytotoxic T lymphocytes (CTLs). These cells are normally prone to CTL-mediated elimination. However, several viruses have evolved strategies to avoid detection by the immune system that interfere with the pathway of antigen presentation. Epstein-Barr virus (EBV) expresses a predominantly late protein, the BCRF1 gene product vIL-10, that is similar in sequence to the human interleukin-10 (hIL-10). We show here that vIL-10 affects the expression of one of the two transporter proteins (TAPs) associated with antigen presentation. Similarly, hIL-10 showed the same activity. Expression of the LMP2 and TAP1 genes but not expression of TAP2 or LMP7 is efficiently downregulated, indicating a specific IL-10 effect on the two divergently transcribed TAP1 and LMP2 genes. Downregulation of TAP1 by IL-10 hampers the transport of peptide antigens into the endoplasmatic reticulum, as shown in the TAP-specific peptide transporter assay, their loading onto empty MHC I molecules, and the subsequent translocation to the cell surface. As a consequence, IL-10 causes a general reduction of surface MHC I molecules on B lymphocytes that might also affect the recognition of EBV-infected cells by cytotoxic T cells.