In situ hybridization was used to identify the cell types infected by hepatitis C virus (HCV) in the liver. Using an antisense HCV-RNA probe from the 5' non-coding region, HCV-RNAs molecules were detected in liver sections of 4/11 patients with chronic hepatitis C. These 4 positive subjects were also infected by HIV. HCV-RNA-positive strands were detected in scattered hepatocytes as well as in cells identified as mononuclear cells within the inflammatory infiltrates. HCV-RNA negative strands, likely replicative intermediates, were also detected in these cells. This study therefore indicates that replication of HCV may occur in both hepatocytes and mononuclear liver cells.