Peripheral blood mononuclear cells (MNC) from three chimpanzees infected with hepatitis C virus (HCV) and from two uninfected animals were analyzed by monoclonal antibody phenotyping using flow cytometry. Significant differences in numbers of MNC's expressing cluster designation (CD) phenotypes CD4, CD14, CD19, and CD45RA were found. Additionally, significant differences in MNC proliferation in response to mitogens were also found. This altered proliferative capacity and cellular phenotype profile may be important markers in studying the pathogenesis of chronic HCV disease.