To study possible extrahepatic sites for the replication of hepatitis C virus (HCV), we examined fresh and cultured peripheral blood mononuclear leukocytes (PBML), as well as different subpopulations of PBML of HCV-infected patients, for the presence of viral genomic and antigenomic RNA. Sense and antisense oligonucleotide primers derived from HCV sequences were used for reverse transcription (RT) followed by an amplification with the polymerase chain reaction assay (PCR). Using antisense primers for RT, genomic viral RNA could be detected in serum, liver, total PBML and B lymphocytes of chronically infected patients. However, only liver tissue and PBML specimens were positive when a sense primer was used. To demonstrate further the specificity of these findings, total PBML were stimulated using pokeweed mitogen and synthesis of HCV RNA was determined by incorporation of [3H]uridine into nascent viral RNA molecules using a hybrid release assay. Additionally, total PBML from an uninfected person could be infected in vitro using an HCV RNA-positive serum. The PCR products obtained from serum, liver and PBML specimens of an HCV-positive individual were found to have nearly identical sequences. Our findings suggest that PBML could be a site for viral replication of HCV during the natural course of infection and may represent a reservoir for hepatitis C virions.