A flow-through optosensor for quinine is described. It has been developed in conjunction with a flow-injection analysis system and uses immobilized beta-cyclodextrin as the sensing agent. The detection limit for quinine was 0.20 ng ml(-1) with a relative standard deviation of 2.1% for the determination of 0.05 mu g ml(-1) (n = 7) of quinine. The method has been successfully tested for the determination of quinine in some pharmaceutical preparations.