Human monoclonal antibodies to rabies virus were established by Epstein-Barr virus infection of peripheral blood lymphocytes collected from a rabies-vaccinated donor, and fusion with a heteromyeloma line. Two human monoclonal antibodies, HUM1 and HUM2, both IgG2, reacted with the envelope glycoprotein of the rabies virus. The antibody HUM1 neutralized rabies virus (lyssavirus serotype 1) and Mokola virus (lyssavirus serotype 3), but did not neutralize European bat lyssavirus, suggesting that some common antigenicity exists between the glycoproteins of serotypes 1 and 3. In addition, this antibody neutralized a series of viruses resistant to neutralization by antibodies recognizing, in a murine system, antigenic sites I, II and III; however, it failed to neutralize viruses altered at site VI, indicating that human monoclonal antibody HUM1 is directed against antigenic site VI. The other human anti-glycoprotein antibody, HUM2, neutralized the European bat lyssavirus in addition to serotypes 1 and 3, but none of the resistant variant viruses altered at the sites mentioned above. A third human monoclonal antibody, HUM3 (IgM), was reactive with the internal nucleoprotein of the rabies virus. This antibody contained a murine light chain corresponding to the cytoplasmic murine chain not secreted in the heteromyeloma line. The potential use of monoclonal antibodies in post-exposure treatment of rabies is discussed.