The interaction between quinidine sulfate (QDS) and congo red (CR) was studied using resonance light scattering (RLS) technique, ultraviolet-visual spectrophotometry and fluorimetry. In weak acidic medium, QDS reacts with CR to form a supermolecular complex which results in the enhanced RLS intensity. Some important interacting parameters, such as the solution acidity and CR concentration, salt effect and addition order of the reagents, were investigated and optimized. Under the optimum conditions, it was found that the enhanced RLS intensity was in proportion to the concentration of QDS in the range 0.2-8.4 microg mL(-1). The corresponding detection limit was 12.0 ng mL(-1). The results showed that this new method enabled simple, sensitive and rapid determination of QDS and was used for the determination of QDS in urine and simulated huamn serum samples.