Fusion of WI-L2-729-HF2 human lymphoblastoid cells and human B-cell blasts provides a very efficient and rapid means of isolating stable B-cell hybridomas that secrete high levels of new human immunoglobulins. By titrating the plating density of fused cells into microwells immediately following fusion, it has been possible to obtain monoclonal hybrids. In this communication, proof of monoclonality is provided based on subcloning, karyotyping, and Southern blot analyses of lambda light chain immunoglobulin genes. The results reveal rearranged lambda genes in hybridoma subclones that produce both kappa (the WI-L2-729-HF2 isotype) and new lambda light chains. In contrast, the WI-L2-729-HF2 parental cell line and kappa-producing hybrids exhibit a germline configuration of lambda genes. The results provide evidence that stable, monoclonally derived hybridomas may be obtained upon initial plating of fused cells, without subsequent subcloning. The data further demonstrate the WI-L2-729-HF2 system to be ideal for rapidly generating, at very high frequency, clonal human B-cell hybridomas that stably secrete human monoclonal antibodies.