Six years after the introduction of serological tests for the detection of antibodies to HCV these diagnostic procedures have been improved significantly. The ELISA's of the third generation contain 4-5 fragments of structural and nonstructural proteins which have increased the sensitivity and specificity for antibody detection. Several confirmatory assays have been established to detect non-specific reactions. Viremia in acute and chronic HCV infection can be detected by RT-PCR. However, studies on the quality of PCR results with serum panels in a large number of diagnostic labs indicate the urgent need to standardize the RT-PCR techniques. Quantification of viral genomes during acute and chronic HCV infection seem to be of major importance to predict the response rat to IFN. Quantitative RT-PCR techniques using internal standards and the branched DNA technology have been established. Techniques for genotyping HCV isolates have been introduced which also seemed to be important for prediction of IFN response. Recently serotyping of HCV isolates has been evaluated and seems to correlate well with genotyping by RT-PCI and specific probes. The detection of quasispecies in the 5'NCR may, in the future, be an additional prospective test for the prognosis of IFN treatment.