We investigated the infection of peripheral blood mononuclear cells (PBMNC) by hepatitis C virus (HCV) in 5 patients with HCV-related chronic hepatitis. The presence of HCV-RNA-positive and -negative strands was tested with the polymerase chain reaction (PCR) method. In all subjects, HCV-RNA was shown in PBMNC. In 3 cases, HCV-RNA was shown in the T- and B-cell populations, with viral RNA also present in the monocyte-macrophage fraction of two of these. HCV-RNA-negative stranded molecules, indicative of the viral multiplication, were significantly increased in cells maintained in cultures with PHA/PMA stimulation. The results indicate that HCV infect blood mononuclear cells, thus suggesting that this cellular tropism may play a role in HCV infection.