In transformation by Epstein-Barr virus of lymphocytes derived from patients with acute hepatitis B, continuous cell cultures were obtained which produced anti-HBc IgM antibodies. These cell lines underwent from 50 to 150 passages. The level of the specific immunoglobulin production was shown to have a trend to decline; however, after cloning the antibody production became more stable. The antibodies produced by the cloned lymphocyte culture could not neutralize the antigen (unlike polyclonal antibodies) which indicated a high efficacy of cloning. When the solid phase was sensitized with produced immunoglobulins, specific activity of binding of HBc antigen detectable by anti-HBc conjugate with horseradish peroxidase was demonstrated. The study of sedimentation properties of antibodies produced by transformed lymphocytes showed their sedimentation constant to be 19S.