Human gene MAGE-1 codes for an antigen that is recognized on a melanoma by an autologous cytotoxic T lymphocyte (CTL). Because MAGE-1 is expressed on a significant proportion of tumours of various histological types and not on normal tissues, the encoded antigen may serve as a target for cancer immunotherapy. Evaluation of the expression of the gene by reverse transcription polymerase chain reaction (RT-PCR) in various tumour samples and tumour cell lines has suggested great variability in the level of expression. It was therefore important to evaluate the minimal level of expression required for lysis by CTL. We tested a number of melanoma cell lines by a quantitative RT-PCR assay to correlate their level of MAGE-1 expression and recognition by the relevant CTL clone. We found that only cell lines expressing more than 10% of the MAGE-1 messenger RNA (mRNA) level of reference cell line MZ2-MEL.3.0 (i.e. more than three mRNA molecules per cell) were lysed by the CTL or induced significant tumour necrosis factor release.