Hypericin, solubilized in an instillation fluid consisting of an aqueous buffer supplemented with 1% plasma proteins, is currently used as a clinical diagnostic tool for the detection of superficial TCC (transitional cell carcinoma) tumors. However, the development of a sterile and stable hypericin stock formulation, excluding the presence of plasma constituents, would be an important factor in a more general clinical application of the method. Therefore, we investigated the stability of several heat sterilized hypericin formulations and ion pairs. Besides sodium hypericinate (in distilled water, in phosphate buffer, in polyethyleneglycol (PEG) 400), several other hypericinate salts (potassium, lysine, TRIS or hexylamine) were investigated. As to that, the physical appearance of different hypericin concentrates stored at 4 and 37 degrees C was investigated. Besides, after dilution into cell culture medium, the ability of hypericin remaining to accumulate in tumor cells and demonstrating photocytotoxic effects upon light irradiation was assessed. These findings suggest that PEG 400 is an excellent hypericin formulation, since it maintained the stability of the compound for at least 120 d when stored at either 4 or 37 degrees C. PEG 400 therefore is a suitable vehicle for the storage of hypericin prior to preparation of the bladder instillation solution.