Abstract: Enzymes such as L-methionine gamma lyase, L-lysine decarboxylase, L-aspartase and L-tryptophanase are immobilized by contacting with a latex of polymer particles having a negative surface charge such that the electrophoretic mobility of the latex has a negative value of -2.0 10.sup.-8 m.sup.2 V.sup.-1 sec.sup.-1 or below when measured at pH 7 in 0.01M KNO.sub.3 and a polymer solids concentration of 30 mg/l. The polymer particles also have a hydrophobic surface such that the contact angle of a 1 microliter droplet of distilled water with a plane horizontal surface formed by drying and compacting the latex particles is 70.degree. or more. The resultant-enzyme/polymer particles can be used for assaying amino acids corresponding to the specific enzyme immobilized in aqueous solutions using an electrosensor comprising a pH-sensitive electrode located in a sensing zone or chamber containing the enzyme/polymer particles as an aqueous dispersion or in water-dispersible form.