Maleinimides are widely used to label proteins and to derivatize thiols. The so-called click reaction was shown to allow the efficient introduction of the maleinimido group into azido-modified fluorophores (benzoxazines) by reacting them with an alkyne-modified maleinimide to yield new thiol-reactive fluorescent labels 5-7. The reaction proceeds under mild experimental conditions and provides a new strategy with which to introduce the maleimide group, as exemplified here for fluorophores. Conceivably, it may be extended to radioactive, electro-active, isotopic, or spin labels. Previously reported methods for the formation of such maleinimides starting from open-ring precursors require rather harsh conditions. The new benzoxazines 5-7 are characterized by a fairly long and flexible linker between the chromophore and the maleinimide functional group, and their fluorescence can be photoexcited with diode lasers (which are preferred light sources in fluorometry). They were conjugated to: (a) the aminothiol cysteamine, (b) the peptide glutathione, and (c) to human serum albumin. The fluorescence of the phenoxazinone 5 is strongly solvatochromic, which suggests its use as a polarity-sensitive probe.