MHC class I antigen expression is necessary for CD8+ T-cell-mediated recognition of tumors. Recently, several mechanisms leading to loss or decreased expression of MHC antigens on the tumor cell surface have been described that may account for tumor escape from immune recognition. It is yet unknown whether tumor recognition by CTL occurs at a threshold amount of MHC molecules or correlates with the level of HLA-allele expression. In this study, a model was developed in which clones derived from the 624-MEL melanoma cell line and expressing varying amounts of HLA-A2 molecules were lysed in a standard 51Cr release assay by an HLA-A2-restricted CTL clone (A42) or a bulk culture of tumor-infiltrating lymphocytes. The A42 clone and the tumor-infiltrating lymphocyte culture were characterized previously as specifically recognizing the melanoma antigen MART-127–35 peptide. A marked heterogeneity in the susceptibility to lysis by A42 was observed in tumor clones and was not due to heterogeneous expression of MART-1 by the clones or loss of accessory molecules involved in the lymphocyte-target interaction. Lysis by A42 and by the tumor-infiltrating lymphocyte culture significantly correlated with the level of HLA-A2 expression, evaluated as mean channel number of fluorescence by flow cytometry (P < 0.001). Transfection of an HLA-A2-negative clone (624.28) with the HLA-A2.1 gene produced a panel of clones expressing different levels of HLA-A2, the lysis of which was highly correlated with the expression of HLA-A2 (P < 0.001). The addition of exogenous MART-127–35 peptide enhanced lysis of clones expressing intermediate amounts of HLA-A2 but did not affect clones with high expression. These data suggest that the number of HLA molecules present on the surface of tumor cells can quantitatively affect their lysis by CTL in situations with borderline amounts of peptide and/or MHC.