PDT in dermatology is of growing interest and there are efforts to develop new sensitizers for topical application. Human keratinocytes and dermal fibroblasts were incubated with different concentrations of Acetoxy-tetrakis-(methoxyethyl)-porphycene (ATMPn) for different times and subsequently irradiated with red light (580-740 nm) of different light doses and intensities to determine photodynamic efficacy using a MTT-assay. First, concentrations ranging from 0 ng/ml to 1,000 ng/ml ATMPn (solved in DMEM) were tested (incubation time 24 h, irradiance 80 mW/cm2, total light dose 60 J/cm2). Second, three different light intensities and doses were used for irradiation (47 mW/cm2/35 J/cm2; 80 mW/cm2/60 J/cm2; 107 mW/cm2/80 J/cm2; incubation time 24 h, 100 ng/ml ATMPn). Third, different incubation times of ATMPn (100 ng/ml, 60 J/cm2) were tested (1, 2, 4, 6, 8, 16, 24 h). Using 1 ng, 10 ng and 25 ng/ml, a PDT-effect could not be seen as compared to the dark control group (100%). A significantly high PDT cell mortality rate (94%) was achieved with 100 ng/ml ATMPn yielding a low dark toxicity rate (11%), results with concentrations between 50 and 150 ng/ml were comparable. However, compared to photofrin which served as control (10 (mu) g/ml), dark toxicity rates were significantly lower (Photofrin: 67%). Using different light intensities and doses, best cell killing results were achieved with 60 J/cm2. Comparing different incubation times, no significant differences in cell killing were found. Thus, optimal in vitro parameters for PDT with ATMPn were established using 100 ng/ml ATMPn, about 1- 8 h incubation time and irradiation with 60 J/cm2. As compared to photofrin in previous studies, this novel sensitizer exhibits reduced dark toxicity and similar phototoxicity at a 100-fold lower concentration.