Many factors are involved in a relatively new tumor treatment modality, the photodynamic therapy (PDT). Among them the most important are the laser parameters (wavelength, energy fluence), the photosensitizer cytotoxic and optical properties, the delay between drug injection and PDT and the effective dosimetry of both light and drug distribution in the treatment target. In this work, we compare the effectiveness of two methods of predicting the concentration of the photosensitizer m-THPC in tissue simulators. In the first method, a conventional double beam spectrophotometer was used to measure the absorption in UV- VIS of the soft tissue simulators, prepared with agar gel with different concentration of m-THPC. In the second method, the laser induced fluorescence spectra of these samples were recorded and an effort was made to determine from these the optical properties of the samples, in order to calculate the required drug concentration in vitro. The experimental results are discussed in conjunction with the appropriate theoretical models.