In this study we set out to investigate the photodynamic efficacy of hypericin (Hyp), three other photosensitizers and 5-aminolevulinic acid-induced protoporphyrin IX in their ability to block the growth of rather aggressive tumor - Ehrlich ascites tumor in mice. Hypericin was found to exhibit the highest tumor growth inhibiting activity in treating Ehrlich ascites tumor by photodynamic therapy. The different photosensitizers were ranked as follows: Hypericin > hematoporphyrin dimethyl ether > Photofrin II > meso-tetra-(para-sulfophenyl) porphin > 5-aminolevulinic acid. Of importance, just after Hyp-based photodynamic therapy 75% of mice survived a 4-month period, and no recurrence of tumor within this period was detected in 25% of the treated mice. The clear cut correlation observed between tumor dye concentration in the tumor cells and efficiency of photodynamic therapy, supports the idea that the intracellular accumulation of the photosensitizer is one of the most important factors in determining the benefit of photodynamic therapy. Hence, the accumulation of the photosensitizer in the tumor cells should be considered as one of the prognostic factors for the determination of the therapeutic outcome.