The quantitation of serum tocilizumab using liquid chromatography tandem-mass spectrometry(LC-MS/MS)method has not been widely applied in clinical settings because of its time-consuming and costly sample pretreatments.The present study aimed to develop a validated LC-MS/MS method for detecting serum tocilizumab by utilizing immobilized trypsin without an immunoglobulin G purification step and evaluate its applicability in the treatment of rheumatoid arthritis(RA)patients administered intrave-nously or subcutaneously with tocilizumab.The tocilizumab-derived signature peptide was deciphered using a nano-LC system coupled to a hybrid quadrupole-orbitrap mass spectrometer.The serum tocili-zumab was rapidly digested by immobilized trypsin for 30 min.The chromatographic peak of the signature peptide and that of the internal standard were separated from the serum digests for a total run time of 15 min.The calibration curve of serum tocilizumab concentration was linear with a range of 2-200 μg/mL.The intra-and inter-day accuracy and relative standard deviation(RSD)were 90.7％-109.4％and＜10％,respectively.The serum tocilizumab concentrations in the RA patients receiving intravenous and subcutaneous injections were 5.8-28.9 and 2.4-63.5 pg/mL,respectively.The serum tocilizumab concentrations using the current method positively correlated with those using the enzyme-linked immunosorbent assay,although a systematic error was observed between these methods.In conclu-sion,a validated LC-MS/MS method with minimal sample pretreatments for monitoring serum tocili-zumab concentrations in RA patients was developed.