Objective To study the expression of MN mRNA in peripheral blood of renal cell carcinoma(RCC) patients and to evaluate its clinical significance. Methods Peripheral blood MN mRNA expression was detected by means of RT PCR 30 patients with renal cell carcinoma,including 16 with localized RCC and 14 with metastatic RCC,in 30 patients without renal tumors series as controts. Results With RT PCR, MN mRNA expressin was detected in 13 of 30 cases of RCC (43%) including 5 of 16 patients(31%) with localized RCC and 8 of 14 patients(57%) with metastatic RCC, none of the 30 control cases being positive. No correlation was noted between MN results and tumor stage. Conclusions RT PCR assay of MN is a highly sensitive technique for detecting circulating renal carcinoma cells in the peripheral blood, and it may be helpful in the diagnosis of micrometastasis.MN can be regarded as a cell specific relative protein of clear cell renal carcinoma.