A novel optical immunoassay system was developed and tested using human IgG as a model antigen. Following adsorption of antiserum to the surface of an optical waveguide, the immobilised antibody was then reacted with a solution containing antigen. The reaction was detected utilising the evanescent wave component of a light beam totally internally reflected within the waveguide. The growing antigeh-antibody layer resulted in an increase of scattered light which was monitored kinetically. Individual measurements were completed within 5 minutes and the resultant dose response curve had a sensitivity limit of approximately 10 nmol/L.