A Mach-Zehnder interferometer is presented, which uses the evanescent field of a planar optical waveguide as the sensing element. Changes in the refractive index profile occurring in the immediate vicinity of the waveguide surface cause the propagation velocity of the light passing through it to change. This is measured interferometrically. Adsorption processes of antibodies onto the waveguide surface as well as immunoreactions have been monitored. Concentrations of 2.5 × 10¿8 M (1 × 10-3 mg/ml) of the antigen HCG (molecular weight = 39 600) have been measured so far.