Optical properties of Talaporfin, a photosensitizer for photodynamic therapy, under the human serum albumin (HSA) existence were investigated using phase-resolved fluorescence. Talaporfin-HSA complex was analyzed in the phosphoric acid buffer solution (PBS) at concentrations ranging from 1.95 x 10(-7) M to 1.00 x 10(-4) M. The fluorescence spectra of Talaporfin PBS solution show a peak around 662 nm, whereas the peak of Talaporfin-HSA complex is red shifted to 670 nm. The photophysical parameters of Talaporfin-HSA complex is attributed to the electronic transition from the lowest singlet excited state to the ground state at concentrations below 1.25 x 10(-5) M. As for the PBS solution of a Talaporfin-HSA complex, its fluorescence intensity is stronger than that of Talaporfin PBS solution. This result could be explained as follows: in the Talaporfin-HSA complex Talaporfin molecules combine to the HSA site II, and hence they feel the less polar circumstance than in the bared PBS solution because of the surrounding amino acid residues of HSA. The fluorescence life time of Talaporfin-HSA complex increases slightly with Talaporfin concentration, and its value of 4.9-5.5 ns is longer than that of the PBS solution.