Abstract: Hence the invention concerns a method which can be used to provide a fluorescent label on bioorganic molecules carrying amino groups such as amino acids, proteins, pharmaceutical agents, antibodies, amino group-modified nucleotides and also polymers and polymer particles carrying amino groups by means of a chemical (covalent) binding. The method is based on the reaction of a pyrylium salt located on a fluorophore F with the amino group of a biomolecule or particle according to the described reaction equation. The method is selective, simple to carry out and results in high labelling yields. The spectral properties of the conjugates differ considerably from those of the starting compounds and their fluorescence quantum yields are often considerably increased.