A monoclonal heterohybridoma cell line (B11-12) was developed by fusing EBV-transformed lymphocytes from a patient with hyper-IgE syndrome and SHM-D33 heteromyeloma cells. The resultant heterohybridoma secreted only human lambda L chains exhibiting specific binding to mouse monoclonal Ab to human IgE (mAb-algE). B11-12 bound to four separate mAb-algE, but not to mAb of the same isotypes directed to other antigens. The mAb-algE differed in isotype (IgG1; IgG2b) demonstrating that B11-12 was recognizing an idiotypic, but not an isotypic determinant on the mAb-algE. Human IgE did not inhibit B11-12 binding to mAb-algE, suggesting the interaction between B11-12 and mAb-algE involved an idiotope outside the combining site of the mAb-algE. Human sera, including serum from the donor of the lymphocytes used to produce heterohybridoma B11-12, demonstrated the capacity to inhibit B11-12 binding to mAb-algE. This, along with the inability of these same sera to directly bind B11-12, suggests that the serum contained reactivity similar to B11-12. We propose that B11-12 demonstrates specificity resembling an anti-Id to anti-human IgE Ab.