In most fluorescent proteins characterized by light absorption and emission in the red and the far-red spectral region (550-650 nm), the chromophore pi system is extended at the expense of the additional oxidation of the GFP-like structure and the formation of an acylimine substituent. As distinct from these proteins, the photoactivateable protein asFP595 contains a chromophore with the keto group substituted for an acylimine substituent. In this work, we studied the reactions that result in a bathochromic shift in the spectrum of asFP595. Maturation kinetics analysis has shown the generation of the immature form containing a protonated chromophore (absorption at 420 nm) at the intermediate step, as in the case of other red fluorescent proteins, which then is isobestically converted into the final mature form (568 nm). Mass spectrometric analysis of the chromopeptide isolated from immature asFP595 has demonstrated that the intermediate form contains a GFP-type chromophore. It has also been found that the oxidation of the GFP chromophore is accompanied by the generation of an equimolar amount of hydro gen peroxide. The intermediate products of oxidation have been analyzed by the mutagenesis of the first chromophore-generating amino acid residue. It has been demonstrated that in the case of all mutants studied, chromophore synthesis does not terminate at the stage of the acylimine derivative, but immediately results in the fragmentation of the main chain of the protein and in the formation of the keto form.