The administration of biological protein therapeutics can lead to an unwanted immune response resulting in the generation of anti-drug antibodies (ADA) with potentially harmful clinical consequences. Hence, to develop safe and efficacious biotherapeutics, the immunogenic potential needs to be examined during the development phase. Current assay technologies measuring ADAs are subject to interference by high circulating concentrations of the protein therapeutic, raising concerns about data reliability since protein therapeutic-free washout samples are not always available. Herein, we report the development and characterization of a magnetic bead based immunoprecipitation method followed by quantitative LC/MS to determine ADA in human and cynomolgus serum in the presence of high circulating concentrations of the protein therapeutic. Available ADA binding sites are saturated by the addition of excess therapeutic followed by magnetic bead based protein G isolation of IgG antibodies and their bound antigens before elution and digestion. Peptides of the target therapeutic proteins are then quantified by LC/MS using stable isotope labeled standards inferring the presence of total ADA. This approach complements established methodologies for the assessment of immunogenicity responses and currently supports clinical programs addressing the safety and tolerability of human growth hormone analogues.