A simple procedure has been developed for the large scale purification of fetal bovine serum acetylcholinesterase (AChE) (EC 3.1.1.7). The procedure involves two steps: batch adsorption of the AChE from 250 L of serum onto a procainamide affinity Sepharose 4B gel; and analytical procainamide affinity chromatography of the step-1 product. Over 100 mg of AChE was purified in 10 days to apparent homogeneity with this procedure.