Two lines of mouse-human hybridomas were fused to produce human bifunctional antibodies. A hybridoma that secretes human IgM (kappa) class antiricin monoclonal antibody was treated with actinomycin D to inhibit cell growth. Another cell line, secreting human IgM (lambda) class antidiphtheria toxin monoclonal antibody, was 6-thioguanine resistant (HAT sensitive). Six hybridomas secreting IgM with both kappa and lambda light chains were obtained. Two of them secreted antibodies with antiricin specificity (kappa and lambda light chains) and antidiphtheria toxin specificity (kappa and lambda light chains). Antidiphtheria toxin antibodies secreted from other hybrid hybridomas did not contain kappa light chain, but lambda chain. A hybrid hybridoma clone 5-29 secreted complete hybrid IgM with lowered affinity to diphtheria toxin. These results suggested unbalanced recombination of immunoglobulin polypeptide chains in hybrid hybridomas.