Organophosphate (OP) pesticides are neurotoxic compounds that are widely used in agriculture. Classical methods for monitoring OP exposure comprise the measurement of intact OP, its metabolites or cholinesterase activity. Newly developed methods focus on the analysis of the OP adduct bound to proteins such as butyrylcholinesterase (BuChE) and albumin. These adducts can be analyzed by means of fluoride reactivation or by analysis with LC–MS/MS of the pepsin or pronase digest of butyrylcholinesterase and albumin, respectively. The utility of these methods is illustrated through the analysis of plasma samples obtained from patients taken 1–49days after ingestion of the organophosphate pesticides chlorpyrifos and/or diazinon. Thus, in this particular case several independent methodologies were applied to the biomedical samples, all pointing to the same exposure.