An optical biosensing instrument has been developed for the quantitative analysis of immunological reactions on biochemically sensitized surfaces. It is based on the measurement of reflectance changes of polarized laser light incident on high refractive index substrates at angles close to the pseudo-Brewster angle. Multiple-angle Brewster angle reflectometry (MABAR) has been used to investigate the binding of anti-human serum albumin (a-HSA) to human serum albumin (HSA) coated silicon surfaces. The concentration dependence and reaction kinetics of antibody-antigen complex formation have been studied using red and green He-Ne laser light. A significant increase in sensitivity has been observed with green light.