We developed a sensitive technique of detecting circulating tumor cells in carcinoma patients, using CD45 immunomagnetic separation to isolate epithelial cells in blood samples and specific polymerase chain reaction analysis to identify point mutations of the K-ras gene. The method is based on the fact that the peripheral blood mononuclear cells (PBMC) that express CD45 antigen are trapped with anti-CD45 conjugated supramagnetic microbeads while the carcinoma cells that do not express CD45 antigen are not trapped and pass through the magnetic fields. This method concentrated the number of carcinoma cells 3.3 times. After this separation, the modified method of mutant allele specific amplification was applied and this method was able to ten control carcinoma cells in a background of 107 PBMC. A preliminary clinical study demonstrated that six cases of end-stage carcinoma with K-ras mutations in the primary tumor showed the same mutations in the peripheral blood samples, while two cases without K-ras mutation in the primary tumor and 10 healthy volunteers showed no mutation in the peripheral blood samples. The results suggest that this method may be very useful to detect circulating carcinoma cells in the patient whose primary tumor shows K-ras mutations.