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 mug mL(-1), except for rituximab (5-100 mug mL(-1)). This method was reproducible, repeatable, and accurate (coefficient of variation and bias < 20%), with no cross contamination. Adalimumab concentrations were significantly higher (p = 0.0198) for patients with biological remission (median: 11.3 mug mL(-1) [4.6; 18.3]) than that for patients without a biological response (9.5 mug mL(-1) [3.94;17.0]). An adalimumab cut-off concentration of 8.0 mug mL(-1) correctly discriminated patients with or without biological remission (sensitivity: 74.1%, specificity: 57.9%). This validated LC-MS/MS routine-suited method is the first allowing simultaneous quantification of up to seven mAbs acting against different pharmacological targets. It opens the field of TDM to numerous mAbs.