FU-266, a mutant human myeloma cell line sensitive to hypoxanthine/aminopterin/thymidine (HAT), was transfected by protoplast fusion with DNA of the recombinant plasmid vector pSV2-neoR, thus acquiring a dominant marker conferring resistance to the antibiotic G-418. One of the resultant neoR clones, E-1, was fused to irradiated (500 rads) or unirradiated cells of the HAT-sensitive, G-418-sensitive, nonproducer mouse myeloma line X63-Ag8.653. Hybrid clones were selected in G-418 plus ouabain, thus preserving their HAT sensitivity. Small numbers of human chromosomes were retained in all such hybrids, but most of them ceased secreting human myeloma (IgE(lambda). Selected hybrid clones were then tested as malignant fusion partners in a series of fusions with polyclonally activated human B lymphocytes and with antigen-primed human B lymphocytes, in some instances after transformation of the latter with Epstein-Barr virus. The yield of viable chimeric hybridomas has been consistently high, as has the proportion of hybridomas secreting human immunoglobulin molecules unpermuted with mouse or human myeloma heavy or light chains. Secretion by many subcloned hybridomas has been stable for over 6 months, and several antigen-specific human monoclonal antibodies have been generated. Thus these heteromyeloma cell lines appear to have significant advantages for human monoclonal antibody production.