The detection of serum HCV-RNA needs to be standardized. The aim of this study was to assess the effectiveness of the branched DNA amplification method in detecting and quantitating serum HCV-RNA in 54 blood donors, 33 with and 21 without increased serum alanine aminotransferase levels and with detectable serum HCV-RNA by polymerase chain reaction. HCV-RNA was detected by branched DNA signal amplification in 42/54 (77%) of the blood donors. Positivity rates were not different among the 21 blood donors with normal and the 33 blood donors with increased serum alanine aminotransferase levels (86% and 76%, respectively). Median serum HCV-RNA levels were not different among donors with or without increased serum alanine aminotransferase levels (28.6 x 10(5) Eq/ml and 14.7 x 10(5) Eq/ml, respectively). There was no significant correlation between serum alanine aminotransferase levels and serum HCV-RNA levels. These findings show that branched DNA signal amplification identifies most of the donors with true hepatitis C virus viremia and that the level of hepatitis C virus replication is not correlated to serum alanine aminotransferase levels.