In spite of the recent progress in molecular biology of the hepatitis C virus (HCV) genome, the biological characteristics of this virus remain poorly known. This is primarily because biological assays for HCV have been limitted to the experimental inoculation of chimpanzees. It is imperative to develop either a less expensive animal model or a cell-culture system for propagating HCV. Several studies, including ours, have provided evidence for replication of the HCV genome in cell cultures. Although the reported systems are not yet fully satisfactory for wide application to in vitro studies of HCV, they are useful at least for examining the infectivity of HCV materials.