An immunoassay method based on the labelling of an antigen with a transition-metal carbonyl organometallic marker and detection of the label by Fourier transform infrared (FT-IR) spectroscopy is described. The viability of this novel non-isotopic approach, termed infrared immunoassay (IRIA), has been evaluated in the quantitative determination of the tricyclic antidepressant nortriptyline.