Rapid and specific determination of the RNA gene of hepatitis C virus (HCV), which had been multiplied by NASBA, was performed using a fluorescence polarization assay. The polarization of the probe DNA in the presence of HCV positive sample, amplified by NASBA, was obviously different from those in the presence of negative control samples. The total time for the gene amplification and detection was about 90 min, while the polarization detection was completed within 10 min. The slight increase of polarization was also confirmed with the hybridization between probe oligo-DNA 25-mers and the synthesized complementary oligo-RNA 25-mers. The polarization of positive and negative samples showed excellent agreement with the results obtained from electrophoresis and dot-blot hybridization.