The monoclonal antibody G250 (mAbG250) raised against a human renal cell carcinoma (RCC) has been shown to react with a large number of RCCs. Recently, G250 antigen was isolated and found to be homologous to the MN/CA9 gene originally identified in HeLa cells. To determine whether G250 antigen (MN/CA IX/G250) could be a potential therapeutic target and a tumour marker, a total of 147 cases of RCC were investigated immunohistochemically as well as by reverse transcriptase polymerase chain reaction (RT-PCR) analysis. In addition, total RNAs extracted from patients' peripheral blood samples were analysed for MN/CA9/G250 mRNA signals. Immunohistochemistry demonstrated strong expression in 128/147 (87.1%) of RCCs, in contrast to the lack of expression observed in normal tissues. RT-PCR analyses of frozen specimens resulted in the clear detection of MN/CA9/G250 mRNA signals in 137/147 (93.2%), and despite subtle differences the results were almost identical to those for immunohistochemistry. Although high-grade and -stage tumours exhibited significantly lower expression than low-grade and -stage tumours, a large proportion of tumours expressed MN/G250 protein as well as mRNA. RT-PCR analysis of patients' blood samples revealed the presence of circulating MN/CA9/G250 expressing cells. These findings suggest that this antigen may be a potential therapeutic target as well as diagnostic marker for RCCs.