Malignant tumor cells can escape CD8+ cytotoxic T cell killing by downregulating class I major histocompatibility complex (MHC) expression. Stable class I MHC surface expression requires loading of the heavy chain/light chain dimer with antigenic peptide, which is delivered to class I MHC molecules in the endoplasmic reticulum by the presumed peptide transporter, encoded by the transporter associated with antigen presentation (TAP) 1 and 2 genes. We have investigated whether loss of class I MHC expression frequently observed in different cancers could result from interference with TAP function. A polyclonal antiserum, raised against a bacterial glutathione S-transferase/human TAP-1 fusion protein, was used for the immunohistochemical analysis of TAP-1 expression in 76 cervical carcinomas. Results showed loss of TAP-1 expression in neoplastic cells in 37 out of 76 carcinomas. Immunohistochemical double staining procedures in combination with HLA-specific antibodies revealed congruent loss at the single cell level of TAP-1 and HLA-A/B expression in 28 out of 37 carcinomas. The remaining samples expressed HLA(-A) in the absence of TAP-1 (n = 6) or showed loss of HLA(-A/B) while TAP-1 was expressed (n = 3). These data strongly indicate that inhibition of peptide transport by downregulation of TAP-1 is a potential strategy of malignant cells to evade immune surveillance.