The parameters of a sandwich assay to analyze syngeneic polyclonal and monoclonal anti-idiotypic antibodies to murine anti-HLA and anti-human melanoma associated antigen monoclonal antibodies have been characterized. Furthermore, the sensitivity of the sandwich assay has been compared with that of assays widely used to characterize anti-idiotypic antibodies, i.e., the binding assay to F(ab')2 fragments of monoclonal antibodies and the radioimmunoassay. The latter is more sensitive than the sandwich assay when monoclonal anti-idiotypic antibodies are tested, but less sensitive when anti-idiotypic antisera are analyzed. Both assays are less sensitive than the binding assay to F(ab')2 fragments of monoclonal antibodies.