Abstract: The invention provides a method and apparatus for monitoring processes in living cells by measuring optical reflectivity by Surface Plasmon Resonance at the surface and/or inside living cells attached to a thin metal film (28), wherein said thin metal film is attached or optically coupled to a base of a prism (27) such that a collimated and optically polarized light beam (6c) in the near-infrared and/or mid-infrared wavelength ranges directed to a side surface of the prism is internally reflected by said prism at its base (27b) and measured by detector means (33) capable of measuring the intensity and optionally also polarization or phase of the reflected beam (6r).