This paper presents a protocol for routine assays of human blood cholinesterase activities which separates erythrocytes from plasma by centrifugation and measures acetylcholinesterase activity in unwashed erythrocytes and butyrylcholinesterase activity in the plasma. The recommended substrate for both enzymes is 1.0 mM acetylthiocholine. The protocol is compared with other two recommended protocols for the activity measurements of the two enzymes using the Ellman method. The paper discusses the advantages and disadvantages of each and concludes with a proposal for an international agreement between laboratories for the evaluation of a standardized protocol.