A human monoclonal antibody, 105AD7, was produced by fusion of a mouse/human heteromyeloma cell line with lymphocytes from a patient previously injected with mouse monoclonal antibody 791T/36 for tumour immunoscintigraphy. The 105AD7 hybridoma has been in continuous culture for more than 12 months, producing a human monoclonal IgG1 which binds to 791T/36 (IgG2b) and its IgG2a class switch variant, but not a range of other monoclonal mouse immunoglobulins. In quantitative flow cytometric assays, 105AD7 was shown to block the binding of fluorescein-labelled 791T/36 to its target gp72 antigen at the surface of tumour cells, but not the binding of 228, an anti-carcinoembryonic antigen (CEA) monoclonal antibody to CEA. Tests with purified 105AD7 antibody demonstrated a stoichiometric high-affinity interaction between 105AD7 and 791T/36. Thus 105AD7 is a human anti-idiotypic antibody which binds at or very close to the binding site of 791T/36, and as such is a candidate for anti-idiotypic immunization of cancer patients.