We used in situ hybridization to detect hepatitis C virus (HCV) infection of peripheral blood mononuclear cells (PBMNC) from 11 patients with chronic active hepatitis. Using 35S-labeled HCV-RNA probe, HCV-RNA-positive and -negative strands were observed in unstimulated PBMNC from three patients, all of whom were receiving immunosuppressive drugs after orthotopic liver transplantation (OLT). HCV-RNA sequences were also identified in PBMNC from three patients who were not undergoing immunosuppression, after stimulation with either phytohemagglutinin (PHA) or pokeweed mitogen (PWM). In contrast, HCV- RNA was not found in the remaining five patients, who had not undergone OLT and whose cells were not stimulated with mitogens. These results show that mononuclear cells can be infected by HCV and that mitogenic stimulation of infected cells increases HCV-RNA replication.