Anti-Ipilimumab Inhibitory Antibodies inhibit the binding of the drug ipilimumab to its target, cytotoxic T lymphocyte-associated antigen 4 (CTLA4), and therefore detect the free drug. An antibody pair is ideal for the development of a pharmacokinetic (PK) bridging ELISA. Three antibodies of high, medium, and low affinity are suitable as a surrogate positive control for an anti-drug antibody (ADA) assay. These recombinant, monoclonal antibodies are generated using Human Combinatorial Antibody Library (HuCAL®) technology and CysDisplay®, a proprietary method of phage display, and use guided selection methods to obtain highly targeted reagents. The result, according to the manufacturer, is highly specific and high-affinity antibodies, ideal for the development of PK assays. HuCAL antibodies are fully human and when in full-length IgG format, can be used as a surrogate positive control or calibrator in an ADA assay.