The study of the molecular biology of infectious agents involves the examination of their genomes and the products of those genomes. Molecular biology methods may therefore allow us to study either DNAs or RNAs. The study of genomic RNAs (viruses) or messenger RNAs (all infectious agents) is used increasingly in infectious disease pathology. The hepatitis C virus was identified in 1989 and was shown to be responsible for a large number of chronic hepatitis cases in France and worldwide. This virus is a good model for the development of technologies to study RNAs, which will later be applied to the study of other viruses. The molecular biology methods used to study hepatitis C virus RNA may be classified into 3 categories. a) Detection methods evidence nucleic acids in fluid or tissue samples, mainly using the polymerase chain reaction (PCR), but also newly developed techniques, such as the NASBA (nucleic-acid-sequence-based amplification), the Q-beta reaction, and the LCR (ligase chain reaction), and techniques that localize viral RNAs in tissue (in situ hybridization, in situ PCR). b) Quantitative methods determine the amount of RNA present in a sample. These include quantitative PCR and new technologies based on signal amplification, such as the 'branched DNA' assays which have recently been developed. c) Qualitative analysis of the genome uses genotyping methods to classify viral strains into different genotypes and subtypes.