To determine the changes in ceramide levels dependent on PDT-dose (a), cells were treated with Photofrin-PDT as described for Fig. 1, except for additional light doses (0.5 and 1.5 J/cm2). For post-treatment time-dependence analysis (b), cells were after PDT light treatment left in the 37°C incubator for either 15 min, 3 hours, or 18 hours before collected for antibody staining and flow cytometry. Using CAL-27 instead of SCCVII cells (c) served to demonstrate the PDT-induced surface ceramide and S1P induction in a different cell line. The post-incubation time interval used with these cells was 3 hours. Ceramide and S1P levels determination was as described for Figs. 1 and 2. The bars denote SD, n = 4; * = statistically significant difference from corresponding untreated group levels, ** = statistically significant difference from 15-min group.