Hepatitis C virus (HCV) continues to represent the main causative agent of the hepatitis, which leads to chronic transformation of the process in 60-80% individuals. It remains unclear how far cellular expression of HCV proteins in vivo may represent an index of progression of the disease and of proliferative activity in the liver in chronic hepatitis C. Aim of the studies included detection and subcellular localization of three HCV proteins (NS3, NS5A and C) in liver biopsies from adults (n=19) with chronic, long lasting hepatitis C as related to hepatocyte proliferative activity. The immunocytochemical ABC (avidin biotin-peroxidase complex) technique was applied, alone or associated with the ImmunoMax technique. Results of the immunocytochemical tests were compared to histological alterations in liver biopsies, proliferation index and with selected clinical data. A significantly higher expression of NS3 protein was noted, as compared to expressions of NS5A and C proteins. In all the patients, cytoplasmic localization of all proteins dominated over nuclear localization (p0.05). At the level of electron microscopy, protein localization in endoplasmic reticulum (ER) membranes, mitochondria, perinuclear region and/or in hepatocyte cell nucleus was observed. No direct relationships could be demonstrated between expressions of HCV proteins and of Ki-67 antigen. No correlations could also be demonstrated between cellular expression of any HCV protein on one hand and grading or staging, alanine transaminase (ALT), serum level of HCV RNA or alpha-fetoprotein (AFP) on the other. However, positive correlations were disclosed between proliferative activity of hepatocytes on one hand and patient's age, grading and staging on the other. Advanced hepatic fibrosis correlated also with serum levels of AFP. The studies were supplemented with data on subcellular localization of HCV proteins. Moreover, they indicated that in HCV infection grading and staging, proliferative activity of hepatocytes and serum AFP level represent more valuable indices of the disease progress than those provided by cellular expression of three potentially oncogenic HCV proteins in vivo.