The combination of Epstein-Barr-Virus (EBV)-permitted immortalization and somatic hybridization (fusion with a myeloma partner) may be the method of choice to produce human monoclonal antibodies. We show here that the fusion of EBV-infected human B-lymphocytes to the HAT-sensitive, ouabain-resistent heteromyeloma (human x mouse) fusion line CB-F7, resulted in stable growing hybridomas producing much more immunoglobulin than the parental lymphoblastoid lines. A more efficient clonability was shown for hybridoma cultures too. The loss of B cell markers (HLA-class II antigen, CD-22, CD-37) was detected. Limiting dilution experiments showed a better fusionability of IgM-producing EBV-transformed B cells in comparison to IgG-secreting counterparts.