The cytotoxic effects of the natural photosensitizing agent hypericin were evaluated. A dramatic difference in the sensitivity of several different human and mouse cell lines towards photoactivated hypericin (4 J cm-2) was demonstrated using a neutral red assay (e.g. A431, IC50 = 0.14 +/- 0.02 microM; HeLa, IC50 = 0.32 +/- 0.05 microM, MCF7, IC50 = 1.84 +/- 0.22 microM). Dark cytotoxicity was absent, even at high hypericin concentration (25 microM). The differential phototoxicity of hypericin did not correlate with the expression of the epidermal growth factor (EGF) receptor nor with the expression of the P170 glycoprotein in the cell. The reduction of the intracellular glutathione content did not enhance further the cytotoxic effects of photoactivated hypericin, as investigated with the A431, HeLa and MCF7 cells. Conversely, using confocal laser microscopy, it was shown that the phototoxicity correlated well with the hypericin cellular uptake.