Serum from an individual who was chronically infected with hepatitis C virus (HCV) was inoculated into chimpanzees and a human T cell line. The serum contained a quasispecies population of HCV consisting of at least six genetic sequences. However, only two sequences (the same two sequences) were recovered both in chimpanzees and in cultured human T cell line, indicating that infection of the culture system reflects infection in vivo.