The photodynamic effects of 5,10,15, 20-tetra(4-methoxyphenyl)porphyrin (TMP) on a Hep-2 cell line were investigated. TMP toxicity in the dark and in relation to illumination with visible light was examined. Hep-2 cells were treated with different TMP concentrations (1, 5 and 10 microM). The uptake of TMP by Hep-2 cells increased with TMP concentration and an increase of the initial uptake rate was observed with increasing TMP concentrations. However, after 24 h of incubation, a similar value of intracellular TMP concentration was reached at all three concentrations of TMP added. Cell toxicity induced by TMP was analyzed in the dark at different concentrations of the photosensitizer and at several incubation periods. The cell mortality obtained after exposure of the cell cultures to visible light was exclusively due to the photosensitization effect of TMP produced by light irradiation. Staining with the hematoxylin-eosin method demonstrated that treatment with TMP, followed by exposure to visible light, notably increased the apoptotic figures. Fas antigen was only expressed in these conditions. The results contribute to the understanding of the photodynamic therapy (PDT) mechanism produced by TMP on Hep-2 carcinoma cell line.