In situ hybridization was used to investigate hepatitis C virus infection in peripheral blood mononuclear cells in 11 patients with chronic hepatitis. Using 35S labeled HCV-RNA probe, HCV-RNA positive and negative strands were observed in unstimulated cells of 3/11 patients; all 3 being treated with immunosuppressive drugs after orthotopic liver transplantation. HCV-RNA sequences were also identified in mononuclear cells obtained from 3 patients without immunosuppression, after stimulation of the cells with either phytohemagglutinin or pokeweed mitogen. In contrast, HCV-RNA was not found in 5 cases, where there was no liver transplantation or cell stimulation by mitogens. These results definitely assess infection of mononuclear cells by HCV. In addition, they demonstrate that mitogenic stimulation of infected cells increases HCV-RNA replication.