A comparative study of the efficiency of procedures for the determination of the biomarkers of organophosphorus agents (OPAs) in blood plasma was performed. It was found that the gas chromatography–mass spectrometry determination of OPAs reactivated from the composition of protein adducts is a rapid method for the detection of exposure to OPAs. The liquid chromatography–mass spectrometry determination of phosphonylated butyrylcholinesterase and albumin fragments modified with OPA residues provides an opportunity to perform more sensitive and retrospective analysis. The tyrosine adducts of OPAs with serum albumin and other blood plasma proteins are not prone to dealkylation in the course of aging; in the series of the test markers, they possess the greatest diagnostic value because they make it possible to determine the precise structure a toxic agent after the longest time interval after exposure. The tentative limit of detection of OPA markers varies from 0.1 to 1.0 ng/mL.