The photosensitizing effects of di-sulphonated aluminium phthalocyanine (AlS2Pc) on proliferation of normal human epidermal keratinocytes of the established line UP were studied in cultures in 96-well microtitre plates using the MTT-microculture tetrazolium assay as a method of assessing viable cell number. It was found that the cytotoxic effects of AlS2Pc were dose-dependent. A cooled slow-scan CCD (charge-coupled device) imaging system with computerized image processing was used for fluorescence measurements in cell cultures after administration of 25 μg ml−1 AlS2Pc. Fluorescence was at a peak at 24 h and this time was therefore considered to be the most appropriate for light sensitization. Treatment with 25 μg ml−1 AlS2Pc for 24 h followed by exposure to total red light (660–700 nm) energy dose of 0.6 J cm−2 reduced cell survival by 100%. It seems that the photokilling capacity of AlS2Pc on keratinocytes is very effective and this may make it particularly suitable for the treatment of surface epithelial tumours.