The organophosphorus (OP) nerve agent soman (GD) contains two chiral centers (a carbon and a phosphorus atom), resulting in four stereoisomers (C+P+, C-P+, C+P-, and C-P-); the P- isomers exhibit a mammalian toxicity that is approximately 1000-fold greater than that of the P+ isomers. The capacity to assess the binding or hydrolysis of each of the four stereoisomers is an important tool in the development of enzymes with the potential to protect against GD intoxication. Using a gas chromatography-mass spectrometry-based approach, we have examined the capacity of plasma-derived human serum albumin, plasma-purified human butyrylcholinesterase, goat milk-derived recombinant human butyrylcholinesterase, and recombinant human paraoxonase 1 to interact with each of the four stereoisomers of GD in vitro at pH 7.4 and 25 degrees C. Under these experimental conditions, the butyrylcholinesterase samples were found to bind GD with a relative preference for the more toxic stereoisomers (C-P- > C+P- > C-P+ > C+P+), while human serum albumin and paraoxonase 1 interacted with GD with a relative preference for the less toxic isomers (C-P+/C+P+ > C+P-/C-P-). The results indicate that these human proteins exhibit distinct stereoselective interactions with GD. The approach described presents a means to rapidly assess substrate stereospecificity, supporting future efforts to develop more effective OP bioscavenger proteins.