A simple colorimetric method for determining serum or plasma cholinesterase has been described. The cholinesterase acts on acetylthiocholine to release thiocholine, which reacts with 5:5-dithiobis-(2-nitrobenzoic acid) to produce a yellow color. The activity is measured after incubation for 3 min. at 37° the reaction is stopped by the excess addition of quinidine sulfate. The difference in absorbance between the blank and test sample is then measured at 412 mµ. Duplicate determinations can be run on 0.1 ml. of serum or plasma, and the method is well suited to laboratories using routine procedures.