A quantitative competitive RT-PCR targeting the specific mRNA of c-erbB-2 is described. It is well known that in many cancers the c-erbB-2 gene dosage is elevated and/or the mRNA is overexpressed leading to a poor prognosis. The recently established method to enrich epithelial cells from peripheral blood made it reasonable and logical to develop a sophisticated competitive RT-PCR method to estimate the c-erbB-2 mRNA load of such cells. By this enrichment method it was discovered that cancer cells can be either isolated as single cells or clusters from patients' blood. The plausibility for their metastatic potential is derived from the measurement of the c-erbB-2 mRNA expression. A SKBR3 cell model suggested that the c-erbB-2 mRNA load is between 168 and 336 molecules per cell. Results of the quantitative competitive RT-PCR method can be used as a rational to determine the efficiency of anti-oncogenic therapies and as a criterion in adjuvant therapy decision.