The determinazione of HCG is performed by the enzyme immunoassay technique using acetylcholinesterase enzyme as lable and a pH electrode to follow the enzyme activity. The antibody to HCG is immobilized in a membrane form with a polyethylene net; to the sample a certain amount of enzyme-labeled HCG is added, the antibody bound membrane is dipped for two hours; after it is taken out, washed and immersed in an acetylcholine Solution and a pH variation can be detected by placing a glass pH electrode with flat surface against the memberane for a few minutes. the pH variation is related tio the HCG concentration of the sample.