The amine reactive succinimidyl ester of TO (thiazole orange) can be conjugated to peptides, proteins, drugs, polymeric materials and biomolecules with primary amine groups. The conjugates are expected to be essentially nonfluorescent until they are able to complex with nucleic acids, resulting in green fluorescence. The conjugates might be useful for studies of nucleic acid binding to various biomolecules, such as DNA-binding proteins. It is also possible that conjugates of other biomolecules might be capable of monitoring their transport into the nucleus. TO dye conjugates of solid or semisolid matrices, such as microspheres, magnetic particles or various resins, might be useful for the detection or affinity isolation of nucleic acids.