An increasing need in the development of biotherapeutic agents is the ability to monitor a potential autoimmune response to the therapeutic target of interest. Unfortunately, the presence of high concentrations of therapeutic antibody can hinder such detection, because there is competition for binding in cases where epitopes are not structurally distinct. This situation was encountered in the development of LY2062430, a therapeutic mid-domain monoclonal anti-amyloid beta peptide (Aβ) antibody undergoing clinical trials for the treatment of Alzheimer's disease. This communication reports the development and validation of a novel radioimmunoassay used to measure potential patient immune responses to Aβ in the presence of LY2062430. This assay employs a radioiodinated analog of the human amyloid beta 1–40 peptide (Aβ1–40) in which a single amino acid substitution of alanine for phenylalanine at position 19 (F19A) effectively eliminates binding by LY2062430. In contrast, F19A binding by monoclonal antibodies specific for the N- and C-termini of the human Aβ1–40 peptide was shown to be unaltered. Additional experiments involving a polyclonal rabbit antibody raised against the midregion of Aβ1–40 (residues 15–30) resulted in only a slight reduction in binding to the F19A tracer, suggesting that the modification does not affect distal epitopes in Aβ1–40 and supporting the notion that this conservative substitution produces only subtle change in the overall peptide structure. The assay is therefore believed to detect most, if not all, patient antibodies to native Aβ peptides. The assay was validated for use in clinical trials allowing detection of antibodies to Aβ in human serum in the presence of therapeutic concentrations of LY2062430.