We have investigated the photoactivating effect of hypericin on two cancer cell lines: PC-3, a prostatic adenocarcinoma non-responsive to androgen therapy and LNCaP, a lymphonodal metastasis of prostate carcinoma responsive to androgen therapy. The two cell lines are incubated for 24 h with hypericin at concentrations ranging from 0.001 to 0.3 microg/ml in cell culture medium. The cells are irradiated at 599 nm (fluence = 11 J/cm2) using a dye laser pumped by an argon laser. Hypericin exerts phototoxic effects on both cell lines, while it does not produce toxic effects in the absence of irradiation. These results suggest that photodynamic therapy (PDT) with hypericin could be an alternative approach to the treatment of prostatic tumors, and could be beneficial in tumors that are non-responsive to androgen therapy.