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RT-PCR is increasingly used for the detection of minimal residual disease in solid tumors. Carcinoembryonic antigen (CEA) RT-PCR seemed to be highly specific for detection of tumor cells when tested on PBMC. A very high frequency of RT-PCR amplification product for CEA in PBSC from breast cancer patients mobilized with G-CSF was found. However, this result contrasted with tumor cell detection by immunocytochemistry (ICC) which showed no correlation with RT-PCR results. In addition, CEA mRNA was amplified in most G-CSF-mobilized PBSC samples derived from patients with hematological malignancies and from healthy donors of allogeneic stem cells, although no circulating epithelial cells could be demonstrated by ICC. CEA RT-PCR expression was observed in PBMC from healthy individuals incubated in vitro with G-CSF. These data suggest that CEA transcription can be induced by G-CSF, resulting in a loss of specificity of CEA RT-PCR for tumor cell detection in PBMC. We conclude, CEA RT-PCR may not be recommended to detect tumor cell contamination in peripheral blood from patients treated with G-CSF. This may have implications on tumor cell detection by RT-PCR in tissues where endogenous or exogenous growth factors may induce the transcription of CEA or other genes.