A method for the identification of serum cholinesterase phenotypes is proposed. It is based on methods already reported (Morrow and Motulsky, J. Lab. Clin. Med. 71: 350-356, 1968; Whittaker et al., Br. J. Anaesth. 53: 511-516, 1981) and uses alpha-naphthyl acetate as substrate and DL-propranolol and RO2-0683 as inhibitors. Its application provides discrimination of the examined phenotypes (CHE1 U, CHE1 UF, CHE1 UA, CHE1 AK, CHE1 AF and CHE1 A). The advantages over other current methods are the following: clear identification of the CHE1 UF phenotype, no need of stringent control of temperature and reaction time, measurements at visible wavelength and adequacy for population studies.