Human monoclonal antibodies (humab) directed against viral antigens were developed by combining immortalization of human primary B lymphocytes by Epstein-Barr virus (EBV) infection with consecutive fusion of selected immortalized lymphoblasts with an established, human lymphoblastoid cell line. Using EBV infection a high rate of immortalized B lymphoblastoid cells was obtained which unfortunately could not be cloned because at least 10 cells per microtiter well had to be seeded to get cells growing. However, fusion of these immortalized lymphoblastoid cells which had been selected for antiviral humab production with an established cell line resulted in hybridomas which could easily be cloned. Among the antiviral humab producing hybridomas thus generated, were three which produced anti-rubella humab. Rubella virus consists of three structural proteins, the core protein C and the two envelope proteins E1 and E2. By the western blot technique we were able to show that two humab reacted with the core protein and the third humab with the envelope protein E1. From the hybridomas grown in stationary cultures, highly purified humab preparations were obtained by subjecting the concentrated culture supernatant to immuno-affinity chromatography.