Detection of the mRNA of selected genes by reverse transcriptase-polymerase chain reaction (RT-PCR) is a sensitive and powerful tool for detecting cancer cells in bone-marrow or peripheral-blood samples. In this study, we determined whether carcinoembryonic antigen (CEA) mRNA is detectable in the peripheral blood of patients with gastrointestinal or breast cancer. In addition, we studied selected patients undergoing surgical procedures to assess whether tumor manipulation during operation enhances cancer-cell dissemination. Peripheral blood from 55 patients with gastrointestinal or breast cancer and from 22 control cases was analysed for CEA mRNA using RT-PCR. For 15 selected cases undergoing curative surgery for cancer, samples were also obtained during and after surgery. The lower limit of detection was 1 to 10 CEA-positive cells diluted among 1 × 107 blood mononuclear cells. The test was positive for 20 of the 55 patients with cancer (36%). None of the 22 control samples were positive. An increase in positivity was observed with increasing stage of disease; however, even some patients with early-stage cancer showed positive results. In addition, CEA mRNA could be detected in the peripheral blood during operation in 3 of 13 patients whose pre-operative CEA mRNA in the peripheral blood had been negative. These findings suggest that, (1) RT-PCR amplification of CEA mRNA is an efficient means of detecting circulating solid cancer cells in the peripheral blood, although long-term clinical studies should be done to evaluate its usefulness; (2) not only breast cancer but also gastrointestinal cancer might be better regarded as a systemic disease even in early stages of carcinoma; and (3) surgical manipulation can provoke cancer-cell dissemination. ©1996 Wiley-Liss, Inc.