When given at two concentrations of 12.5 and 25 microg/kg to mice weighing 18-20 g in chronic Infection, the novel immunomodulator Stimforte was tested for effects on replication of hepatitis C virus (HCV), 1b strain. The efficacy of the agent was evaluated from the decrease in virus titers in the liver, serum, brain, and spleen and from the reduction of antigen titers in the same organs. When administered at a concentration of 12.5 microg/kg, the agent was ineffective and did not decrease significantly the examined indices in any of the organs. When used at a concentration of 25 microg/kg, Stimforte significantly lowered the number of virus antigen in the study organs, rather than in the serum, the liver showing a 15-fold antigen reduction as compared with the controls. HCV replication decreased by 2.4 log10 in the serum and 1.7-1.9 log10 in the organs of the animals given the agent. A Stimforte-induced decrease in HCV replication correlated well with the increased concentration of proinflammatory cytokines (tumor necrosis factor-alpha, interferon-gamma, interleukin (IL)-6, IL-1 beta , which might be one of the mechanisms responsible for the antiviral activity of the agent in hepatitis C.