AIM To study the detection of micrometastasis in peripheral blood of patients with gastric carcinomas andits clinical significance.METHODS A cytokeratin 19 (CK19)-specific nested reverse transcriptase-polimerase chain reaction (RT-PCR) assay was developed to detect CK19 expressing cancer cells, the sensitivity was determined by serialdilution method using CK19 expressing gastric cancer cells, the specificity was assessed by examining 12negative controls and 12 positive controls. Then pre-operative peripheral blood from 42 patients with gastriccancer was detected and the relationship between positive results and biological behavior was studied.RESULTS CK19mRNA was expressed in all the 12 gastric cancer tissues but not in peripheral blood from12 healthy individuals；sensitivity of nested RT-PCR amplification for CK19mRNA was confirmed to be 1/106 by serial dilution method using human gastric cancer line SGC-7901; micrometastases in pre-operativeperipheral blood were detected in 13 (30,9%) patients with gastric carcinomas, the frequency ofmicrometastasis in peripheral blood was significantly correlated with tumor size,depth of invasion and TNMstage (x2 test, P