A new system for the rapid production at high frequency (greater than 10(-5] of stable human hybridomas is described. This system is based on a high efficiency fusion variant human lymphoblastoid cell line designated WI-L2-729-HF2 and is comparable in many ways to commonly used murine hybridoma systems. WI-L2-729-HF2 cells fuse at high frequency with mitogen-stimulated human b cells resulting in the rapid appearance (within 2 weeks) of stable hybridomas in hypoxanthine/aminopterin/thymidine (HAT) medium. Whereas the WI-L2-729-HF2 cell line secretes only trace levels (50 to 100 ng/ml) of an IgG,kappa and has surface IgM,kappa, HAT-resistant hybridomas secrete high levels (10 to 20 micrograms/ml) of new human immunoglobulins, including immunoglobulins containing alpha or lambda chains not found in the tumor parental cell line. These hybridomas have remained stable for over four months in continuous culture, secreting high levels of new immunoglobulins in both conventional and serum-free medium, a feature which makes possible large-scale production and purification of human antibodies. This system has the potential to provide tools and reagents for investigations involving, for example, the diagnosis and treatment of human autoimmune disease and cancer.