Classical plasma butyrylcholinesterase (BChE) purification involves dialysis and multiple steps of chromatography. We describe a procainamide affinity gel purification scheme that takes 15-30 min to purify BChE from 1 ml plasma. The method uses a microfuge spin column to build a 0.2 ml procainamide affinity column. The eluted BChE contains 3-4 microg of 500-fold purified BChE, free from 99% of contaminating plasma proteins. The BChE was further purified by gel electrophoresis. Tryptic peptides from the BChE containing gel electrophoresis band were prepared by in-gel digestion, separated by reverse phase liquid chromatography and identified by mass spectrometry. The 29 residue active site tryptic peptide labeled with the nerve agents soman or sarin was identified.