The photobleaching of meta-tetra(hydroxyphenyl)chlorin mTHPC) (irradiation wavelength 413 nm) in protein-containing solution was evaluated by decay in absorbance in Soret band and in fluorescence (λexc= 423 nm, λem= 655 nm). Light exposure resulted in a decrease in absorption throughout the spectrum and simultaneous appearance of new absorption bands in the spectral region 325–450 nm. The rate of mTHPC photodegradation, followed by decay in absorbance, was 15-fold lower than that observed in fluorescence. This fact reflects the photobleaching of presumably monomeric, fluorescing species of mTHPC. In order to determine the consequences of photobleaching of fluorescing mTHPC material on cellular uptake and photocytotoxicity, human HT29 colon adenocarcinoma cells were incubated with photobleached mTHPC during 5 h with or without following irradiation with the fixed fluence. Surprisingly, but up to the time when the fluorescence decreased by 50%, only a slight decrease in photocytotoxicity was detected. Either aggregated forms that have been taken up undergo intracellular monomerization (but we did not observe increase in fluorescence in living cells) or the photodynamic activity is mostly due to aggregates. The discrepancy of m-THPC-photodynamic therapy (PDT) effect and fluorescence measurements may suggest that aggregated m-THPC plays an important role in mTHPC-PDT.