The interaction of antigen (Ag) and antibody (Ab) with poly diallyldimethylammonium chloride (PDDA) in aqueous solutions has been studied by optical absorption and resonance light-scattering (RLS) spectroscopies. The formation of the three-component-complex is due to aggregates of Ab or Ag with PDDA by electrostatic interaction and aggregates of Ab with Ag by immunoreaction. The influences of some experimental factors, including incubation time, pH value, concentration of PDDA and concentration of Ab, on the aggregation process have also been studied. A linear relationship between the concentration of Ag and the RLS intensity was found. Under the optimal conditions, for a given concentration of Ab (4.6 microg/mL), the enhancement of RLS intensity is in proportion to the concentration of Ag in the range 0.03-0.83 microg/mL. The RLS could, in combination with immunoassay, be a rapid and sensitive detection method for Ag.