A murine retrovirus-infected human T-cell line, HPB-Ma, supported replication of hepatitis C virus (HCV) at least as well as the previously reported Molt4-Ma cells. Cloning of HPB-Ma cells revealed a clonal variation of cellular susceptibility to HCV infection. Using one of the sensitive clones, we tested HCV inocula from different sources for their infectivity titer in cell culture. The in vitro titers obtained correlated with the reported infectivity titers of the inocula in chimpanzees. Thus, the system appears to be useful for estimating the in vivo infectivity of HCV.