The use of therapeutic monoclonal antibodies (mAbs) is steadily increasing. Previous studies have reported the clinical interest of mAb therapeutic-drug monitoring (TDM), including that of adalimumab, for patients with Crohn's disease (CD). Proof of concept mAb-quantification studies by liquid chromatography mass spectrometry (LC-MS/MS) have been published, but a specific and reliable routine-suited multiplex quantification method is still needed to facilitate mAb TDM. We describe an electrospray ionization LC-MS/MS method for the simultaneous quantification of seven mAbs (adalimumab, cetuximab, infliximab, rituximab, secukinumab, tocilizumab, and trastuzumab) in human plasma. Sample preparation was performed using protein-G purification and trypsin digestion to obtain proteotypic peptides. We retrospectively measured the adalimumab concentration in 65 plasma samples from 56 CD patients and determined the adalimumab therapeutic cut-off concentration associated with biological remission. Calibration curves were linear from 1 to 100 μg mL