UC 729-6, a 6-thioguanine-resistant human lymphoblastoid B-cell line, was fused with normal and malignant human lymphocytes. Parent UC 729-6 cells were diploid with a 21p+ marker chromosome, expressed surface and cytoplasmic IgM kappa, and doubled every 17 hr. The resulting human-human hybridomas were pseudotetraploid containing the 21p+ marker, doubled every 20-30 hr, and secreted 3-9 micrograms of human Ig per ml per 10(6) hybrid cells for greater than 9 months in continuous culture. Human Ig-secreting hybridomas were generated in 88% (14/16) of the fusions carried out and were cloned by limiting dilution (one cell per three wells) without the use of feeder layers. The mean fusion frequencies (number of wells, plated at 10(5) cells per well, showing hybrid growth per 10(6) lymphocytes fused) of UC 729-6 with normal lymphocytes ranged from 0.45 to 2.9 and with malignant B lymphocytes, from 3 to 10. Analysis of the human-human hybridomas derived from lymphocytes isolated from regional draining lymph nodes of cancer patients revealed several that secreted human monoclonal antibody that reacted by an enzyme immunoassay with some carcinoma cell lines but not with normal fibroblast cell lines. These data suggest that (i) UC 729-6 can be fused with human lymphocytes to generate stable human-human hybridomas, some of which secrete antibody reactive to human cell surface antigens, and (ii) UC 729-6 can be used to rescue Ig from nonsecretory malignant B cells and thereby allow for the production of anti-idiotype antibodies.