A novel, proteomics based method was developed for the detection, quantification, and categorization of serum butyrylcholinesterase (BChE) inhibitors, including organophosphates (OPs) and carbamates (CBs). This method was based on the MALDI-TOF-MS analysis of the trypsin generated BChE active site peptide (191-SVTLFGESAGAASVSLHLLSPR-212) previously modified by reaction with an OP or CB. The ionization efficiency of OP modified active site peptides by MALDI was greatly improved by adding diammonium citrate to the MALDI matrix, which made the quantification of OP exposure feasible. Excellent linearity (r2 > 0.98) between the normalized abundance ratios (NARs) and OP concentrations or logarithm of carbaryl concentration was obtained. The accuracy of the developed assay was evaluated by comparison of IC50 and IC100 values from the assay with those determined by the Ellman method. Results from this method were comparable with those from the Ellman method. The advantage of the assay was that both the origin and the extent of pesticide exposure can be determined in one analysis. Our MALDI method can provide critical evidence for the pesticide exposure at low BChE inhibition levels even down to 3%, not available with the Ellman method.