Stilbene compounds of the formula ##STR1## in which the rings A, B and C can carry non-chromophoric substituents, R.sub.1 is hydrogen, alkyl which is unsubstituted or substituted by non-chromophoric substituents, or aryl which is unsubstituted or substituted by non-chromophoric substituents, and R.sub.2 is a carboxyl group or a functional derivative thereof, a cyano group, a sulphonic acid ester group, a sulphonamide group or a sulphonyl group, and the use thereof as fluorescent brightening agents for high-molecular organic material.